Question

calculate how many atoms of hydrogen will H3PO4 contain.

The H3PO4 contains 1.2e23 molecules by itself

Answer 1

a molecules of the compound H3PO4 contains 3 hydrogen atoms,
one phosphorus atom, and four oxygen atoms: the molar mass if obtained by adding up the average atomic
masses of these atoms:
molar mass H3PO4 = 3(1.01 g) + 1(30.97 g) + 4(16.00 g) = 97.99g/mol

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Answer 2

if there are 1.2e23 molecules, and there are 3 H atoms per molecule, then multiply the number of molecules by 3.