More Examples for Mass-Mass
Steps in Mass-Mass Problems
1. Write a balanced equation from the known facts about the reaction.
1.5 Identify your known and unknown, draw an arrow going from your known
amount to your unknown amount.
2. Convert the mass of known substance to moles.
3. Use the mole ratio to convert moles of known substance to moles of required
substance.
4. Convert the moles of desired substance to grams.
Example 1 ______________________________________________________________
Calculate the number of grams of HCl needed to react with 10.0 grams of Zn.
Solving Process:
(a) Begin with the balanced equation.
Zn(c) + 2HCl(aq) ZnCl2(aq) + H2(g)
10g xg
(b) Convert grams of zinc to moles
10.0 g Zn 1 mol Zn
65.4 g Zn
...
(c) Determine the mole ratio that exists between Zn and HCl. 1 mole Zn reacts with 2
moles HCl
10.0 g Zn 1 mole Zn
2 mol HCl ...
65.4 g Zn
1 mol Zn
(d) Convert moles of HCl to grams HCl.
10.0 g Zn
1 mol Zn
65.4 g Zn
2 mol HCl
1 mol Zn
36.5 g HCl
1 mol HCl
= 11.2 g HCl
Note that the conversion ratios are chosen and arranged so all the units divide out except
the desired unit (in this case, grams of HCl). Note that since all the ratios are equal to 1,
multiplying by one of them, or by all of them, only changes the units in which the answer
is stated.
Example
2____________________________________________________________________
How many grams of calcium metal will react completely with 10.0 grams of
water to yield calcium hydroxide and hydrogen gas?
Solving Process:
(a) Begin with a balanced equation for the reaction.
Ca(c) + 2H2O(l) Ca(OH)2(c) + H2(g)
(b) Convert mass of water to moles of water.
1 mol H2O
10.0 g H2O
18.0 g H2O
(c) Determine the mole ratio that exists between water and calcium metal.
1 mol Ca reacts with 2 moles H2O
1 mol H2O
1 Mol Ca
10.0 g H2O
18.0 g H2O
2 mol H2O
(d) Convert the moles of Ca to grams
10.0 g H20
1 mol H2O
1 mol Ca
18.0 g H2O
2 mol H2O
40.1 g Ca
1 mol Ca
= 11.1 g Ca
Hence, 10.0 Grams of H20 will react completely with 11.1 grams Ca.
Example
3____________________________________________________________________
Calculate the grams of O2 produced if 2.50 grams of KClO3 are decomposed
completely by heating.
Solving Process:
(a) Write a balanced equation for the reaction.
2KClO3(c) 2KCl(c) + 3O2(g)
(b) Convert mass of KClO3 to moles.
2.50 g KClO3
1 mol KClO3
123 g KClO3
(c) Determine the mole ratio that exists between KClO3 and O2.
2 moles KClO3 yield 3 moles O2
1 mol KClO3
3 mol O2
2.50 g KClO3
123 g KClO3
2 mol KClO3
(d) Convert moles O2 to grams.
2.50 g KClO3
1 mol KClO3
123 g KClO3
3 mol O2
2 mol KClO3
32.0 g O2
1 mol O2
= 0.976 g O2
Hence, 2.50 grams KClO3 decompose to yield 0.976 g O2.
Calculations Involving Impure Substances
Raw chemicals sometimes are not pure. They contain the starting substance plus
another inert material. In any problem involving such raw chemical either the percentage
of pure substance must be known or the actual amount of impure material must be known
so that the actual mass of the pure substance can be calculated.
Example 4 _____________________________________________________________
A sample of Fe2O3 has a mass of 50.0 grams but contains by mass only 80.0%
pure Fe2O3. What is the mass of pure Fe2O3 in the 50.0 grams sample?
Solving Process:
Convert the percentage (80.0%) to a decimal (0.800) and multiply this decimal by
the mass of the sample.
Grams pure Fe2O3 = (50.0 x 0.800) = 40.0 grams pure Fe2O3
Example 5______________________________________________________________
A sample of PbS has a mass of 30.0 grams and contains 10.0% inert material.
What is the mass of pure PbS?
Solving Process:
There are two way of working this problem
1. Subtract the 10.0% inert material from 100.0% to obtain 90.0%,
which is the percentage of pure PbS. Then multiply as before.
Grams of pure PbS = (30.0 x 0.900) = 27.0 grams
pure PbS
2. Multiply by the percentage of inert material and obtain the
grams of inert material. Subtract the mass of the inert material
from the mass of the sample to obtain the mass of pure material.
grams inert material =(30.0 x 0.100)
= 3.00 grams inert material
Grams pure PbS
= (30.0 – 3.00) = 27.0 grams
pure PbS
Example 6_____________________________________________________________
Zinc chloride is produced by the reaction of HCl with Zn. If 40.0 grams of an HCl
solution containing 65.0% water reacts with an excess of Zn, how much ZnCl2 will be
produced?
(a)
Write a balanced equation.
Zn(c) + 2 HCl(aq) ZnCl2(aq) + H2(g)
(b)
Convert the 35.0% HCl solution to pure solution.
40.0(0.350) g HCl
1 mol HCl
36.5 g HCl
1 mol
ZnCl2
2 mol HCl
136 g ZnCl2
= 26.1 g
ZnCl2
1 mol ZnCl2
Hence, 40.0 grams of 35.0% pure HCl solution will react with excess Zn to give
26.1 grams ZnCl2.