Question

What is the molar mass of a compound if 5.19*10^16 molecules of the compound weigh 6.205 microgrammes?

Answer 1

The molar mass of your compound is `"72.0 g/mol"`.

Since molar mass is expressed as the weight of 1 mole of a compound, you must determine exactly how many moles you've got.

Since 1 mole of a substance contains exactly `6.022 * 10^(23)` molecules of that substance - this is known as Avogadro's number, you can use this to figure out how many moles you have

`5.19 * 10^(16)cancel("molecules") * "1 mole"/(6.022 * 10^(23)cancel("molecules")) = 8.618 * 10^(-8)"moles"`

Now just divide the weight you were given by the number of moles you have to get the molar mass

`M_M = "m"/"n" = (6.205 * 10^(-6)"g")/(8.618 * 10^(-8)"moles") = "72.0 g/mol"`

SIDE NOTE Notice I've converted micrograms to grams in order to get the molar mass expressed in g/mol.