11

Chemical Reactions in Aqueous Chemical Reactions in Aqueous SolutionsSolutions

How do we know whether a reaction occurs? What observations indicate a reaction has occurred?

In your groups, make a list of changes that indicate a chemical reaction has occurred.

22

33

Ag+ + Cl- AgCl

Pb2+ + CrO42-

PbCrO4

Cr3+ + 3OH- Cr(OH)3

returnreturn

44

Ba(OH)2.8H2O + NH4Cl

Thermite: Al + Fe2O3

returnreturn

Cesium in a bathtub

Brainiac Thermite movie

2Al + 3Br2 2AlBr3

55

2Na + Cl2 2NaCl

P4 + 5O2 P4O10

returnreturn

(NH4)2Cr2O7(s) Cr2O3(s) + 4H2O(g) + N2(g)

CuSO4.5H2O CuSO4 + 5H2O(g)

2NI3 N2 + 3I2

66

Heat CuSO4

NI3

returnreturn

77

Na + 2H2O NaOH + H2(g)

Cu + 2H+ Cu2+ + H2(g) Mg + 2HCl

MgCl2 + H2

returnreturn

88

Cu + 2Ag+ Cu2+ + 2Ag Zn + Sn2+ Zn2+ + Sn

returnreturn

99

Cesium in a bathtub

Exploding WhaleBuilding Demolition returnreturn

Substances behave differently when they are placed in water, specifically ionic versus co- valent compounds.

One breaks apart in water, the other does not.

Which one is more likely to be pulled apart by water molecules?

Electrolytes are ionic and strong acid solutions (e.g., GatoradeTM); Nonelectrolytes are covalent compounds (e.g., sugar); weak electrolytes are in between.

Strong/WeakElectrolytes

Strong electrolyte: substance that, when dissolved in water, results in a solution that can conduct electricity (NaCl)

Weak electrolyte: substance that is a poor conductor of electricity when dissolved in water (CH3COOH – vinegar)

Nonelectrolyte: substance that doesn’t conduct electricity when dissolved in water (CH3OH – methanol)

Strong/Weak Electrolytes

3 substances: A2X, A2Y, and A2Z. Which one is the strongest electrolyte? The weakest? (Water has been omitted for clarity.)

Most reactions in general chemistry take place in an aqueous environment. What does that mean?

Terms:◦ Solution: homogeneous mixture of two or more

substances◦ Solute: substance present in smaller amount◦ Solvent: substance present in greater amount◦ Aqueous solution: solvent is water

Three general categories:◦ Precipitation: insoluble (solid) product is formed

from aqueous solutions◦ Acid-base neutralization: acid and base react to

form water and a salt (ionic compound)◦ Oxidation-Reduction: electrons are transferred

between atoms in reaction Combination Decomposition Single-replacement (metal or hydrogen)

Precipitation reactions always begin with two ionic compounds.

Example: NaCl (aq) + AgNO3 (aq) ? Draw these compounds in two separate

aqueous environments. What are the possible products when they are combined?

+

Ag+ NO3-

Ag+

Ag+

NO3-

NO3- Cl-

Cl-

Cl-

Na+

Na+

Na+

Write formulas of products (based on charges), predict phases (Solubility Rules), and balance the equation.

If not covered by the rules, it is probably If not covered by the rules, it is probably insolubleinsoluble.. 1919

Determine if the following ionic compounds will be soluble (aq) or insoluble (s) in water:

K2CO3

BaSO4

PbI2 NaClO4

Ag2S (NH4)3PO4

Cu(OH)2

Determine if the following ionic compounds will be soluble (aq) or insoluble (s) in water:

K2CO3 soluble (aq) BaSO4 insoluble (s) PbI2 insoluble (s) NaClO4 soluble (aq) Ag2S insoluble (s) (NH4)3PO4 soluble (aq) Cu(OH)2 insoluble (s)

There are 3 ways to represent ppt reactions: ◦ As whole compounds (molecular equation)◦ As ionic species (ionic equation) – more accurate◦ As participants in reaction (net ionic equation)

Any aqueous ionic substance is written as a compound (e.g., AgNO3), but this isn’t accurate. What does this look like in water?

Solids, liquids, and gases remain as compounds.

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Molecular equation:NaCl(aq) + AgNO3(aq) AgCl(s) + NaNO3(aq)

Ionic equation (write separate ions for soluble (aq) compounds):Na+(aq) + Cl-(aq) + Ag+(aq) +NO3

-(aq) AgCl(s) + Na+(aq) + NO3

-(aq) Net ionic equation (cancel any identical

ion on both sides of the equation, called spectator ions): Ag+(aq) + Cl-(aq) AgCl(s)

Note: s, l, and g stay together!!!!!

Ag+/Cl- ions

Chemistryhumor, ha ha!

Reaction of lead (II) nitrate and potassium iodide. What is the precipitate?

Write the molecular, ionic, and net ionic equations.

Acid: substance that breaks apart in water to form H+ (e.g., HCl, HNO3, CH3COOH, lemon, lime, vitamin C).◦ HA(aq) H+(aq) + A-(aq)

Base: substance that breaks apart in water to form OH- (e.g., NH3, DranoTM, Milk of MagnesiaTM)◦ MOH(aq) M+(aq) + OH-(aq)

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acid base

weak acidstrong acid

You need to KNOW these!!!

Strong acids: HCl, HBr, HI, HClO4, HClO3, H2SO4, HNO3

Strong bases: LiOH, KOH, NaOH, RbOH, CsOH, Ca(OH)2, Sr(OH)2, Ba(OH)2

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What does “neutralization” mean? How might these two solutions react when

combined?

+OH-

Na+

Na+

OH-

H+

H+ Cl-

Cl-

Neutralization reaction: reaction between acid and base; products are salt (ionic compound) and water

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) Acid + base salt + water What are the ionic and net ionic equations

for these reactions? HF(aq) + NaOH(aq) KOH (aq) + H2SO4 (aq)

Acid + Base → H2O + Salt

2HCl(aq) + Mg(OH)2(s) → 2H2O(l) + MgCl2(aq)

Water is created from H+ and OH-

The salt is created from spectator ions

HCl

Mg(OH)2

Complete and balance these equations. Write ionic and net ionic equations, if applicable.

Na2S(aq) + CuCl2(aq) MgSO4(aq) + BaCl2(aq) KNO3(aq) + CaCl2(aq) CuSO4(aq) + NaOH(aq) HF(aq) + NaOH(aq) KOH(aq) + H2SO4(aq)

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If not covered by the rules, it is probablyIf not covered by the rules, it is probably insoluble insoluble.. 3232

Na2S(aq) + CuCl2(aq) 2 NaCl(aq) + CuS(s)◦ Cu2+(aq) + S2-(aq) CuS(s)

MgSO4(aq) + BaCl2(aq) MgCl2(aq) + BaSO4(s)◦ Ba2+(aq) + SO4

2-(aq) BaSO4(s)

2 KNO3(aq) + CaCl2(aq) 2 KCl(aq) + Ca(NO3)2(aq)◦ No reaction

CuSO4(aq) + 2 NaOH(aq) Cu(OH)2(s) + Na2SO4(aq)◦ Cu2+(aq) + 2 OH-(aq) Cu(OH)2(s)

HF(aq) + NaOH(aq) H2O (l) + NaF (aq)◦ H+ (aq) + OH- (aq) H2O (l)

2KOH(aq) + H2SO4(aq) 2H2O (l) + K2SO4 (aq)◦ H+ (aq) + OH- (aq) H2O (l)

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Determine the products of the reaction. Identify the phase of each compound, and balance the equation. Also write the ionic and net ionic equations.

Molecular: Na2S + Cr(NO3)3

Complete Ionic:

Net Ionic:

Oxidation-Reduction (redox) reactions: electron-transfer reactions

When iron rusts, it loses electrons to form a cation, oxygen gain electrons to form an anion: 4 Fe(s) + 3 O2(g) 2 Fe2O3(s)

Use oxidation number rules to determine gain and loss of electrons.

Oxidation numbers are assigned as if elements in compounds completely transferred electrons (like in ionic compounds).

1) An atom (or molecule) in its elemental state has an oxidation number of 0.

2) An atom in a monatomic ion (Na+, Cl-) has an oxidation number identical to its charge.

3a) Hydrogen has an oxidation number of +1, unless it is combined with a metal, in which case it has an oxidation number of –1.

3b) Oxygen usually has an oxidation number of -2. Oxygen in peroxides (O2

2-) has an oxidation number of -1.

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3c) Halogens usually have an oxidation number of -1 (except when bonded to oxygen or in polyatomic ions).

4) The sum of oxidation numbers is 0 for a neutral compound and is equal to the net charge for a polyatomic ion. (Example: NaCl = 0, SO4

2- = -2)◦ 4a) For binary ionic compounds, the position

of the element in the periodic table may be useful:

◦ Group IA: +1; Group IIA: +2; Group VIIA: –1; Group VIA: –2; Group VA: –3

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H2SO4

H = +1; O = –2 S is unknown, so leave this for last.

◦ The overall charge on this compound is 0.◦ Use algebra to solve for S:◦ 2(+1) + 1(x) + 4(-2) = 0

Solve for each element: MgCr2O7

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Determine values of the oxidation number of each element in these compounds or ions:

H2O SO2

CCl4 H2O2

Fe3(PO4)2 MnO4-

NaNO3 KClO4

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Determine values of the oxidation number of each element in these compounds or ions:

H2O H: +1, O: -2

SO2 S: +4, O: -2

CCl4 C: +4, Cl: -1

H2O2 H: +1, O: -1

Fe3(PO4)2 Fe: +2, P: +5, O: -2

MnO4- Mn: +7, O: -2

NaNO3 Na:+1, N:+5, O: -2

KClO4 K: +1, Cl: +7, O: -24040

Oxidized: atom, molecule, or ion becomes more positively charged◦Loss of electrons is oxidation (LEO)

Reduced: atom, molecule, or ion becomes less positively charged (reduced charge)◦Gain of electrons is reduction (GER)

Or: OIL RIG (oxidation is loss; reduction is gain)

Redox 2Redox 1

The substance oxidized causes the other substance to be reduced and is called the reducing agent.

The substance reduced causes the other substance to be oxidized and is called the oxidizing agent.

4 Fe(s) + 3O2(g) 2Fe2O3(s)

Identify the element or ion oxidized/reduced. Also identify the oxidizing agent and the reducing agent.

Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)

Oxidized

Reduced

Identify the oxidation number of each element in the compounds or ions below:

Ba(ClO3)2

SO32-

For the reaction below, identify what has been oxidized and reduced; identify the oxidizing agent and the reducing agent.

Cu(s) + 2AgNO3(aq) 2Ag(s) + Cu(NO3)2(aq)

Combination (1 product)◦ Na(s) + Cl2(g)

Decomposition (1 reactant) – usually give off gases◦ CuCO3(s)

Single Replacement (or Displacement) (start and end with an element and a compound)◦ Zn(s) + HCl(aq)

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Metals tend to react with water to form bases:◦ 2Na (s) + 2H2O 2NaOH + H2

◦ MgO (s) + H2O Mg(OH)2

Nonmetals tend to react with water to form acids:◦ 2F2 (g) + 2H2O 4HF + O2

◦ CO2 (g) + H2O H2CO3

Alkali metals+ water

CO2 (s) + H2O (l) H2CO3 (aq)

element + element compound◦ H2(g) + O2(g)

metal + nonmetal ionic compound◦ Na(s) + Cl2(g)

nonmetal + nonmetal covalent compound◦ C(s) + O2(g)

Why are these redox reactions?

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Mg + O2 K + Cl2

Compound 2 elements; element + compound; or 2 compounds

Oxides, peroxides ◦ Give off O2

Nitrates ◦ Give off NO2 , NO2

-

Carbonates◦ Give off CO2

Ammonium salts ◦ Give off NH3

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NH4Cl (s) NiCO3(s) CuO(s)

element + compound compound + element(The more metallic/active element in the compound is displaced.)

Metal Displacement◦ Zn(s) + Cu(NO3)2(aq) Cu(s) + Zn(NO3)2(aq)

Hydrogen Displacement◦ Mg(s) + HCl(aq) MgCl2(aq) + H2(g)

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Alkali Metals Zn+SnCl2

5454

The higher the metal on the activity series, the more active that metal.

Translation: higher metals on the chart will form ions as products.

5656

Na + H2O Fe + H2O Fe + Cr(NO3)2

Ni + Pb2+

Ag + Mg2+

Zn + Co2+

5757

React w/ coldwater

React w/ steam

React w/ acid

Four metals (A, B, C, and D) are tested and the following observations are recorded.

A + B+ no reaction C + B+ reaction occurs D + A+ reaction occurs D + B+ no reaction

Burning hydrocarbons CxHyOz + O2 (g) CO2 (g) + H2O (g)

Methanol, CH3OH _CH3OH (l) + _O2 (g) _CO2 (g) + _H2O (g)

Precipitation: use Solubility Rules (AB + CD AD + CB)

Acid-Base Neutralization: acid + base salt + water (AB + CD AD + H2O)

Combination: start with elements (A + B AB)

Decomposition: often produces gas (AB A + B)

Single Displacement: use Activity Series to predict if a reaction occurs (A + BC B + AC)

Combustion: ◦ Hydrocarbon + O2(g) CO2(g) + H2O(g)

+

Na Na Na

NaNa Na

Cl Cl

ClCl Cl Cl

+

Ca Ca Ca CaCa Ca Ca

H+

Cl-

Cl-

Cl-

Cl-

H+

H+

H+

+

Cl-

H+

K+

OH-

OH-K+

H+

Cl-

1. Co(s) + AgNO3(aq)

2. Fe(s) + HCl(aq) 3. Na2CO3(s)

4. Ca(s) + H2O(l) 5. CaCO3(s)

6. HClO4(aq) + KOH (aq)

7. BaCl2(aq) + Na2SO4(aq)

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8. HgO(s) 9. LiOH(aq) + H2SO4(aq) 10. Na2CrO4(aq) + Ni(NO3)2(aq) 11. Li(s) + O2(g) 12. Mg(OH)2(aq) + 2HCl(aq) 13. NH3(g) + HCl(g) 14. NiCO3(s) 15. Ca(s) + F2(g)

Single disp. 1. Co(s) + 2AgNO3(aq) Co(NO3)2(aq) + 2Ag(s)Single disp. 2. Fe(s) + 2HCl(aq) FeCl2(aq) + H2(g)Decomp. 3. Na2CO3(s) Na2O(s) + CO2(g)Single disp. 4. Ca(s) + 2H2O(l) Ca(OH)2(aq) + H2(g)Decomp. 5. CaCO3(s) + heat CaO (s) + CO2(g)Acid-base neut. 6. HClO4(aq) + KOH(aq) KClO4(aq) + H2O(l) Precip. 7. BaCl2(aq) + Na2SO4(aq) BaSO4(s) + 2NaCl(aq) Decomp. 8. 2HgO(s) 2Hg(l) + O2(g)

Acid-base neut. 9. 2LiOH(aq) + H2SO4(aq) Li2SO4(aq) + 2H2O(l)Precip. 10. Na2CrO4(aq) + Ni(NO3)2(aq) 2NaNO3(aq) + NiCrO4(s)Combo. 11. 4Li(s) + O2(g) 2Li2O(s)Acid-base neut. 12. Mg(OH)2(aq) + 2HCl(aq) 2MgCl2(aq) + H2O(l)Combo. 13. NH3(g) + HCl(g) NH4Cl(s)Decomp. 14. NiCO3(s) NiO(s) + CO2(g)Combo. 15. Ca(s) + F2(g) CaF2(g)

Predict products of the following reactions. Write correct phases for the products and

balance each equation: ___ Al (s) + ___ NaNO3 (aq) ___ Na (s) + ___ O2 (g) ___ Na2SO4 (aq) + ___ Pb(NO3)2 (aq)

Sections 8.4-8.5

We use the balanced chemical equation to determine relative amounts of reactants and products.

How can we predict the amount of product we should get?

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Mass to mass conversions Given mass of reactant, find mass of precipitate: MgCl2 + 2 AgNO3 2 AgCl + Mg(NO3)2

1.2482 g MgCl2 and excess AgNO3

◦ Convert grams of MgCl2 to moles using molar mass

◦ Convert moles of MgCl2 to moles of AgCl using mol-mol ratio

◦ Convert moles of AgCl to grams using molar mass

NaOH + Al(NO3)3 Al(OH)3 + NaNO3

10.54 g xs ?

How many grams of aluminum hydroxide can be made by starting with 10.54 grams of sodium hydroxide? (Hint: Is the equation balanced?)

The mass calculated is the theoretical yield (the most that can theoretically be made in lab).

If you ran that reaction in lab and only obtained

4.9548 g (actual yield) of Al(OH)3, what percent of the maximum did you obtain?

Percent yield = Actual / Theoretical x 100%

Theoretical will be calculated; actual yield will be given in a problem (or obtained in lab)

Analogy: making martinis I have 7 martini glasses, 36 olives, 12 shots

of vodka, and 8 stirrers Each martini requires 1 glass, 2 olives, 2

shots of vodka, and 1 stirrer How many martinis can I make?

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Limiting Reactant

7474

How would you define limiting reagent?

7575

2H2 + O2 2H2O

What is the limiting reactant?

7676

2H2 + O2 2H2O

What is the limiting reactant?

2H2 + O2 2H2O

What is the limiting reactant?

7878

2H2 + O2 2H2O

What is the limiting reactant?

7979

2H2 + O2 2H2O

What is the limiting reactant?

8080

2H2 + O2 2H2O

What is the limiting reactant?

8181

Al(s) + Cl2(g) AlCl3(s) Begin with 1.67 g Al (s) and 1.67 g Cl2 (g). Calculate mass of product that can be made

from each reactant? 1) Which reactant produces less product? 2) How many grams of AlCl3(s) will be produced,

theoretically? 3) What is the percent yield if you obtained 1.76

g AlCl3 in the lab? 4) How many grams of the excess reagent will

be left over?

Hydrogen and chlorine react to yield hydrogen chloride. How many grams of HCl are formed from reaction of 3.56 g of hydrogen and 8.94 grams of chlorine?

Sections 9.5-9.6

Consider the reaction:◦ 3CaCl2(aq) + 2Na3PO4(aq) Ca3(PO4)2(s) +

6NaCl(aq) If we mix 25.0 mL of 0.200 M CaCl2

solution with 50.0 mL of 0.250 M Na3PO4 solution, what mass of precipitate is formed?

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Complete the equation below (with phases) and balance it.

K2S(aq) + AgNO3(g)

If you combine 10.21 mL of 0.152 M K2S with 1.0092 g AgNO3, how much solid product can be formed?

If 0.2744 g of product were actually formed, what is the percent yield?

Titration: method of determining the concentration (or volume) of an unknown solution by using a solution with a known concentration (standard)◦ moles = M * V

Equivalence point: point at which acid has completely reacted with and been neutralized by base (moles acid = moles base)

End point: point at which the indicator changes color (slight change in solution’s pH)

Titrant in buret (usually) Unknown in flask (usually) Can’t use dilution calculation! Need mol-mol ratio of acid-base Solution stoichiometry: use molarity

to convert between volume and moles (instead of molar mass)◦ M = mol / L

23.78 mL of 0.2500 M NaOH neutralized 20.00 mL of HCl. What is the concentration of HCl? Start with a balanced equation.

Worked Ex. 3.14, Problems 3.20, 3.21, 3.22

How many mL of 1.018 M H2SO4 are needed to neutralize 20.00 mL of 0.9989 M NaOH?◦ Write balanced equation◦ Determine number of moles of known◦ Calculate mole ratio to determine moles of

unknown◦ Divide by concentration to find volume

23.48 mL of lithium hydroxide are required to neutralized 15.39 mL of 1.20 M phosphoric acid. What is the concentration of base?

Concentration: amount of solute present in a given amount of solution

Molarity: moles of solute in 1 L of solution◦ moles solute / liters solution; M = mol / L

1.00 M NaCl = 1.00 mol NaCl in 1 L solution

Knowing concentration, volume, and/or mass allows us to calculate: ◦ 1) mass of solid needed to make a solution of a

certain concentration, or ◦ 2) concentration of solution if mass and volume

are known.

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1.00 M NaCl = 1.00 mol NaCl in 1 L solution

0.500 M NaCl = 0.500 mol NaCl in 1 L sol’n.

To find moles of solute: M * V = mol How many moles in 5.67 mL of 0.500

M NaCl? What volume is needed to make a

0.15 M solution using 5.67 mol of solute?

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What is the concentration of Cl- ions in a 0.150 M FeCl3 solution?

1 mol FeCl3 1 mol Fe3+ + 3 mol Cl-

0.150 M FeCl3 x (3 mol Cl- / 1 mol FeCl3) = 0.450 M Cl-

We can calculate concentration (molarity) if we know mass and volume.◦ Mass solute ÷ molar mass = moles solute◦ Moles solute ÷ volume solution = Molarity

Or we can calculate the mass of solute needed to make a solution of certain concentration (molarity).◦ Molarity * volume solution = moles solute◦ Moles solute * molar mass = mass solute needed

What is the concentration of a 1.00 L solution made by dissolving 5.00 g NaCl in water?

5.00 grams NaCl in a 500.0 mL solution? What mass of NaCl do we need to make 1.00 L of

a 0.500 M solution?

Dilution: lowering the concentration of solution (by adding water)◦ number of moles of solute stays the same,

amount of solvent increases The number of moles of solute stays the

same; volume increases (lower molarity)◦ moles = M * V◦ Mconc * Vconc = Mdil * Vdil or M1V1 = M2V2

Example: What is the final concentration of solution made by adding 100.0 mL of 3.00 M HCl to 300.0 mL of DI water?

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How would you prepare 500.0 mL of 0.500 M H2SO4 starting with concentrated (18.0 M) solution? (Hint: What variable are you solving for?)◦ Remember to add acid to water.

How much water do you need to add to 25.0 mL of a 4.50 M NaOH solution to make a 1.00 M NaOH solution?

1) 13.9 mL of 18.0 M H2SO4 2) 87.5 mL of water added to 25.0 mL of 4.50 M NaOH

Sections 5.8-5.10

Percent by mass: percent by mass of each element in a compound

H2O2

◦ 2 moles H, 2 moles O◦ H = 2 mol x 1.008 g/mol H = 2.016 g H ◦ O = 2 mol x 16.00 g/mol O = 32.00 g O◦ H2O2 = 2.016 g H + 32.00 g O = 34.02 g

◦ %H = 2.016 g H / 34.02 g H2O2 x 100% = 5.926%

What is %O? Solve 2 ways…..

What is the percent by mass of Cr, S, and O in Cr2(SO4)3?

Cr: (104 / 392.21) * 100 = 26.52% S: (96.21 / 392.21) *100 = 24.53% O: (192 / 392.21) * 100 = 48.95%

We can reverse this calculation. Know percentages, need to calculate

empirical formulas.

Calculate the empirical formulas:◦ 50% S, 50% O◦ 43.64% P, 56.36% O

Assume 100 g of the material:◦ 50 g O x 1 mol / 16.0 g = 3.125 mol O◦ 50 g S x 1 mol / 32.066 g = 1.559 mol S◦ mol O / mol S = 3.125 / 1.559 = 2.004 or 2◦ SO2

Assume 100 g of material: ◦ 43.64 g P mol P (1.4089 mol P)◦ 56.36 g O mol O (3.5225 mol O)◦ Ratio of two moles (2.5 O / 1 P); need whole numbers

103103

We just found the empirical formula to be P2O5. This compound has a molecular mass of 141.9 g/mol.

We know the molecular formula has a molar mass of 283.8 g/mol. How many multiples of P2O5 do we need to reach a mass of 283.8?

A compound contains 30.4 % nitrogen and 69.6 % oxygen. The molecular mass of the compound is 92 g/mol.◦ What is the empirical formula of the compound?◦ What is the molecular formula of the compound?

105105

A compound contains 40.00% C, 6.71% H, and the rest is O. Determine the empirical formula of this compound.

If the molar mass of the substance is 180.16 g/mol, determine the molecular formula of this substance.