Unit 5 Quiz Review

Question

Show your work for the following problem: 

Find the number of moles in 742.2g of CaBr₂.

		=

Please answer and explain.

Answer 1

To find number of moles you first have to find the molar mass of the compound. This can be found on the periodic table. For example the molar mass of H2O (water) is 2 hydrogens with mass 1.01 and 1 oxygen with mass 16.00. Therefore the molar mass of water is 18.02 g/mol.

So #1...find the molar mass of CaBr2

Next, we need an equation to convert grams into moles. I write mine like this...

? moles = mass (in this case 742.2g) x 1 mole / the molar mass (which you will calculate)

Molar mass is usually written in grams/mole but it can also be written as moles/gram! It just depends what you are trying to calculate.

Example H2O again:

We have 10 grams H2O, calculate moles

? moles = 10 grams H2O x 1 mole / 18.02 g H2O = 10 x 1 mole / 18.02 = 0.55 moles H2O

notice the unit "grams" cancels and we are left with # of moles.

Hope this helps!