http://www.ck12.org Chapter 10. The Mole
Practice Problem- Find the masses of the following amounts.
a. 2.15 mol of hydrogen sulfide, H 2 S
b. 3.95× 10 −^3 mol of lead(II) iodide, PbI 2
A similar conversion factor based on molar mass can be used to convert the mass of a known substance to moles.
In a laboratory situation, you might perform a reaction and produce a certain amount of a product. It will often
be necessary to then determine the number of moles of the product that was formed, but this cannot be measured
directly. However, you can use a balance to measure the mass of the product, and the number of moles can be easily
calculated. The next Sample Problem illustrates this situation.
Sample Problem 10.5: Converting Mass to Moles
A certain reaction produces 2.81 g of copper(II) hydroxide, Cu(OH) 2. Determine the number of moles produced in
the reaction.
Step 1: List the known quantities and plan the problem.
Known
- mass of Cu(OH) 2 produced = 2.81 g
Unknown
- amount of Cu(OH) 2 produced in moles
One conversion factor will allow us to convert from mass to moles.
Step 2: Calculate.
First, it is necessary to calculate the molar mass of Cu(OH) 2 from the molar masses of Cu, O, and H. The molar
mass is 97.57 g/mol.
2 .81 g Cu(OH) 2 × 971 .mol Cu(OH) 57 g Cu(OH)^2
2= 0 .0288 mol Cu(OH) 2Step 3: Think about your result.
The molar mass is approximately 100 g/mol, so a quick estimate can be obtained by dividing the original value by
100 (moving the decimal point two places to the left). The relatively small mass of product formed results in a small
number of moles.
Practice Problem- Calculate the number of moles represented by the following masses.
(a) 2.00× 102 g of silver
(b) 37.1 g of silicon dioxide, SiO 2