How do you find formula mass?

1 Answer
anor277
Jun 28, 2016

Got a Periodic Table handy?

Explanation:

Consider the formula, and sum up the masses of each element in the formula to give the formula mass. You get an answer in #g*mol^-1, "a mass in gram per Avogadro's number of atoms"#.

So let's take an example: #OsO_4# (I don't know why I chose that one!). In #1# #mol# of this stuff, there are #1# #mol# metal atoms, and #4# #mol# of #O# atoms. I take the masses from the Periodic Table, and I get a mass of #{190.23(Os)+4xx15.999(O)}*g*mol^-1=254.23 *g*mol^-1#.

You can of course do this with any given formula. You will have access to a Periodic Table in any Chemistry or Physics examination.

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