Solution Stoichiometry
Sample problem: Finding the Minimum Volume to Precipitate
Aqueous solutions that contain silver ions are usually treated with chloride ions to recover silver
chloride. What is the minimum volume of 0.25 mol/L magnesium chloride, MgCl2(aq), needed to
precipitate all the silver ions in 60 mL of 0.30 mol/L silver nitrate, AgNO3 (aq)? Assume that
silver chloride is completely insoluble in water.
Overview:
V (AgNO3) = 60 mL = 0.06 L
V (MgCl2) = ?
Molarity (AgNO3) =0.30 mol/L
Molarity (MgCl2) = 0.25 mol/L
Solution
(i) Write a balanced equation: MgCl2 (aq) + 2AgNO3 (aq) → 2AgCl (s) + Mg(NO3)2 (aq)
(ii) Find the amount (mol) of silver nitrate from the volume and concentration of solution.
0.06 L x 0.30 mol = 0.018 mol AgNO3
1L
(iii) Use mole ratios from the coefficients in the equation to determine the amount (in mol) of
magnesium chloride that is needed.
0.018 mol AgNO3 x 1 mol MgCl2 = 0.0090 mol MgCl2
2 mol AgNO3
(iv) Use the amount (in mol) of magnesium chloride and the concentration of solution to find the
volume that is needed.
0.0090 mol MgCl2 x 1 L = 0.036 L = 0.04 L = 40 mL
0.25 mol
Sample problem # 2: Suppose 12.0g of zinc metal is to be reacted with 0.800 mol/L (M) HCl:
Zn (s) + 2HCl (aq) ZnCl2 (aq) + H2 (g) What volume, in L, of HCl will be needed?
Overview:
mol/L
mass (Zn) = 12.0 g
V (HCl) = ?
Solution:
Step 1: change info given (g) into (mol)
Concentration (molarity) HCl = 0.800
12.0g Zn x 1 mol Zn = 0.1835 mol Zn
65.38g Zn
Step 2: find mol of substance required (mole ratio)
HCl
0.1835 mol Zn x 2 mol HCl = 0.3670 mol
1 mol Zn
Step 3: determine volume of substance required
Complete questions p.304:
0.3670 mol HCl x 1 L HCl = 0.459 L HCl
0.800mol
15. Food manufacturers sometimes add calcium acetate to puddings and sweet sauces as a
thickening agent. What volume of 0.500 mol/L calcium acetate, Ca(CH3COO)2(aq), contains
0.300 mol of acetate ions?
16. In a neutralization reaction, hydrochloric acid reacts with sodium hydroxide to produce
sodium chloride and water: HCl(aq) + NaOH(aq) → NaCl(aq)+ H2O What is the minimum volume
of 0.676 mol/L HCl that is needed to combine exactly with 22.7 mL of 0.385 mol/L aqueous
sodium hydroxide?
17. Sodium sulfate and water result when sulfuric acid, H2SO4, reacts with sodium hydroxide.
What volume of 0.320 mol/L sulfuric acid is needed to react with 47.3 mL of 0.224 mol/L
aqueous sodium hydroxide?
18. Your stomach secretes hydrochloric acid to help you digest the food you have eaten. If too
much HCl is secreted, however, you may need to take an antacid to neutralize the excess. One
antacid product contains the compound magnesium hydroxide, Mg(OH)2.
(a) Predict the reaction that takes place when magnesium hydroxide reacts with hydrochloric
acid.
(b) Imagine that you are a chemical analyst testing the effectiveness of antacids. If 0.10 mol/L
HCl(aq) serves as your model for stomach acid, how many litres will react with an antacid that
contains 0.10 g of magnesium hydroxide?