How do you calculate the mass..involve Avogadro's number?

i need help solve these problems..appreciate 4 your help!

Calculate the mass of the following. Avogadro's number = 6.022E23 particles/mol.

a] silver in 22.7 g AgBr
b] nitrogen in 7.14 mol (NH4)3PO4
c] oxygen in 6.35E+22 molecules SO3

Answer:
a. AgBr = 107.9 + 79.9 = 187.8
Silver's proportion is 107.9 / 187.8 = 57.5%.
57.5% of 22.7 g = 13.0 g

b. (NH4)3(PO4) = 133 g/mol
Three nitrogens = 42
Proportion of nitrogen = 42/133 = 31.6%
7.14 mol x 0.316 = 2.25 mol of nitrogen (N)
2.25 mol x 14 g/mol = 31.6 grams of N

c. SO3 = 32 + 48 = 80 g/mol
6.35E+22 molecules / 6.02E23 = 0.105 mol
That's 0.105 mol x 80 g/mol = 8.44 g of SO3
Oxygen's proportion = 48/80 = 60%
60% of 8.44 g = 5.06 g of oxygen.

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