This document discusses how to calculate molar masses of compounds by adding together the atomic masses of each element in the compound. It provides examples of calculating the molar mass of (NH4)2S, P4O10, SiCl4, and Cu3(PO4)2. The document also explains how to perform calculations involving moles, mass, and numbers of molecules or formula units of compounds using dimensional analysis and molar mass values. Practice problems are included for calculating moles or mass given the other value.

1. Formula Masses and Conversions

2. Purpose
• You previously learned about the masses on the periodic table and how to
use them to convert between number of atoms, moles and mass of a
sample. We will now do the same thing for compounds.

3. Molar Masses of Compounds
• Previously we looked at atomic masses for atoms. For example, we said that 1
mole of carbon has a mass of 12.011 g. For compounds, we simply add up the
masses of all the atoms present in the compound.
– Example: What is the molar mass of (NH4)2S?
• First determine how many of each atom are present:
– 2 Nitrogen
– 8 Hydrogen
– 1 Sulfur
• Multiply the number of each atom by their molar mass:
– 2 x 14.01g = 28.01 g
– 8 x 1.01g = 8.08 g
– 1 x 32.07g = 32.07 g
• Add the masses:
– 28.01 g + 8.08 g + 32.07 g = 68.16 g
• The molar mass of (NH4)2S is 68.16 g/mol

4. Pause and Practice
• What are the molar masses of the following?
– P4O10
– SiCl4
– Cu3(PO4)2

5. Pause and Practice
• What are the molar masses of the following?
– P4O10 – 4 phosphorus, 10 oxygen
• 4 x 30.97 g + 10 x 16.00 g = 283.88 g/mol
– SiCl4 – 1 silicon, 4 chlorine
• 1 x 28.09 g + 4 x 35.45 g = 169.89 g/mol
– Cu3(PO4)2 – 3 copper, 2 phosphorus, 8 oxygen
• 3 x 63.55 g + 2 x 30.97 g + 8 x 16.00 g = 380.59 g/mol

6. All masses in grams are for 1 mole and
6.022 x 1023 molecules
• As with atoms – the mass from the periodic table is for one mole of the
substance , which is 6.022 x 1023 individual molecules of the substance
– P4O10
• 283.88 g = 1 mole = 6.022 x 1023 molecules
– Cu3(PO4)2 – 3 copper, 2 phosphorus, 8 oxygen
• 380.59 g = 1 mole = 6.022 x 1023 formula units
• Note: For P4O10 we used the term molecules and for Cu3(PO4)2 we used the
term formula units. The reason for this is because P4O10 is covalent and exists
as individual molecules, while Cu3(PO4)2 is ionic and exists as a large
combination of ions in a 3:2 ratio.

7. Conversions with compounds is similar to conversions
with individual atoms – use dimensional analysis.
• How many molecules of P4O10 are in 34.6 g of P4O10?
– 283.88 g = 1 mole = 6.022 x 1023 molecules
34.6 𝑔 𝑃4 𝑂10
6.022 𝑥 1023 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠𝑃4 𝑂10
283.88 𝑔𝑃4 𝑂10
= 7.34 𝑥 1022 molecules P4O10
• What is the mass of 2.45 mol Cu3(PO4)2?
– 380.59 g = 1 mole = 6.022 x 1023 formula units
2.45 𝑚𝑜𝑙 𝐶𝑢3 𝑃𝑂4 2
380.59 𝑔𝐶𝑢3 𝑃𝑂4 2
1 𝑚𝑜𝑙𝐶𝑢3 𝑃𝑂4 2
= 932 𝑔𝐶𝑢3 𝑃𝑂4 2

8. Pause and Practice
• How many moles of SiCl4 are in 34.2 g SiCl4?
• What is the mass of 4.78 x 1024 molecules of SiCl4?

9. Pause and Practice
• How many moles of SiCl4 are in 34.2 g SiCl4?
– 169.89 g SiCl4 = 1 mol SiCl4 = 6.022 x 1023 molecules SiCl4
34.2 𝑔 𝑆𝑖𝐶𝑙4
1 𝑚𝑜𝑙 𝑆𝑖𝐶𝑙4
169.89 𝑔 𝑆𝑖𝐶𝑙4
= 0.201 𝑚𝑜𝑙 𝑆𝑖𝐶𝑙4
• What is the mass of 4.78 x 1023 molecules of SiCl4?
– 169.89 g SiCl4 = 1 mol SiCl4 = 6.022 x 1023 molecules SiCl4
4.78 𝑥 1023
𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒
169.89 𝑔 𝑆𝑖𝐶𝑙4
6.022 𝑥 1023 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒 𝑆𝑖𝐶𝑙4
= 135 𝑔 𝑆𝑖𝐶𝑙4

10. Try the exercises
• Take your time and ensure you have your units working out correctly.