Some chemistry questions

Question

1. How many molecules of NH_4NO_3 are there in 20grams of NH_4NO_3?
2. What is the mass in grams of a sample of H_3PO_4 if it contains 1.39x10^23 molecules?
3. A sample of MgCl_2 contains 9.74x10^23 molecules of MgCl_2. What is the mass of the sample in grams?

Answer 1

1.) molar mass NH₄NO₃ = 80 g/mol

20 g x 1 mol/80 g x 6.02x10²³ molecules / mole = 1.5x10²³ molecules of NH₄NO₃

2.) molar mass H₃PO₄ = 98.0 g/mol

1.39x10²³ molecules x 1 mol/6.02x10²³ molecules x 98.0 g/mol = 22.6 g H₃PO₄

3.) molar mass MgCl2 = 95.2 g/mol

9.74x10²³ "molecules" x 1 mol/6.02x10²³ "molecules" x 95.2 g/mol = 154 g MgCl₂

(note: when working with ionic compounds like MgCl₂ we refer to formula units and not molecules)