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The question:
How many atoms of hydrogen are there in 560cm^3 of hydrogen at s.t.p.?

My answer:
22,400cm³ at s.t.p. = 1 mole
560cm³ at s.t.p. = 560 ÷ 22,400 moles
= 0.025 mol^-1
1 mole = 6 x 10²³ molecules
0.025 moles = 0.025 x 6 x 10²³ molecules
= 0.15 x 10²³ molecules
2 x 0.15 x 10²³ (2 atoms in H₂) = 3 x 10²²

Is this right? I googled it and someone on another forum said that the answer is  6.02x10²³.

[Borek]

I googled it and someone on another forum said that the answer is  6.02x10²³.

That's just Avogadro's number.

Your answer looks (almost) OK. Just be sure you have used correct number of significant digits. (Not that it matters much).

[Cheerios]

Thanks Borek. You're always so helpful!