Engineers use conservation of mass, called

1. Engineers use conservation of mass, called a “mass balance,” to determine the amount of product

that can be obtained from a chemical reaction. Mass balance assumes that the total mass of

reactants is equal to the total mass of products. Is this a chemically valid practice? Explain your

answer.

2. Given the equation 2H2(g) + O2(g) → 2H2O(g), is it correct to say that 10 g of hydrogen will react

with 10 g of oxygen to produce 20 g of water vapor?

3. What does it mean to say that a reaction is stoichiometric?

4. When sulfur is burned in air to produce sulfur dioxide, what is the limiting reactant? Explain your

answer.

N U M E R I C A L P R O B L E M S

Please be sure you are familiar with the topics discussed in “Essential Skills 2” before proceeding

to the Numerical Problems.

1. What is the formula mass of each species?

a. ammonium chloride

b. sodium cyanide

c. magnesium hydroxide

d. calcium phosphate

2. What is the molecular or formula mass of each compound?

a. potassium permanganate

b. sodium sulfate

c. hydrogen cyanide

d. potassium thiocyanate

3. How many moles are in each of the following?

a. 10.76 g of Si

b. 8.6 g of Pb

c. 2.49 g of Mg

d. 0.94 g of La

4. How many moles are in each of the following?

a. 8.6 g of CO2

b. 2.7 g of CaO

c. 0.89 g of KCl

e. 2.5 g of NaOH

5. Convert the following to moles and millimoles.

a. 1.68 g of Ba(OH)2

b. 0.792 g of H3PO4

h. 5.852 g of sulfuric acid

7. What is the mass of each substance in grams and milligrams?

a. 5.68 mol of Ag

b. 2.49 mol of Sn

13. Under the proper conditions, ammonia and oxygen will react to form dinitrogen monoxide (nitrous

oxide, also called laughing gas) and water. Write a balanced chemical equation for this reaction.

Determine which reactant is in excess for each combination of reactants.

a. 24.6 g of ammonia and 21.4 g of oxygen

b. 3.8 mol of ammonia and 84.2 g of oxygen

c. 3.6 × 1024 molecules of ammonia and 318 g of oxygen

d. 2.1 mol of ammonia and 36.4 g of oxygen

15. Determine the mass of each reactant needed to give the indicated amount of product. Be sure that

the chemical equations are balanced.

a. NaI(aq) + Cl2(g) → NaCl(aq) + I2(s); 1.0 mol of NaCl

b. NaCl(aq) + H2SO4(aq) → HCl(g) + Na2SO4(aq); 0.50 mol of HCl

c. NO2(g) + H2O(l) → HNO2(aq) + HNO3(aq); 1.5 mol of HNO3

16. Determine the mass of each reactant needed to give the indicated amount of product. Be sure that

the chemical equations are balanced.

a. AgNO3(aq) + CaCl2(s) → AgCl(s) + Ca(NO3)2(aq); 1.25 mol of AgCl

b. Pb(s) + PbO2(s) + H2SO4(aq) → PbSO4(s) + H2O(l); 3.8 g of PbSO4

c. H3PO4(aq) + MgCO3(s) → Mg3(PO4)2(s) + CO2(g) + H2O(l); 6.41 g of Mg3(PO4)2

18. Each step of a four‐step reaction has a yield of 95%. What is the percent yield for the overall

reaction?

19. A three‐step reaction yields of 87% for the first step, 94% for the second, and 55% for the third.

What is the percent yield of the overall reaction?

21. Under certain conditions, the reaction of hydrogen with carbon monoxide can produce methanol.

a. Write a balanced chemical equation for this reaction.

b. Calculate the percent yield if exactly 200 g of methanol is produced from exactly 300 g of

carbon monoxide.