calculating hydrogen atoms

Question

how many hydrogen atoms in 3c7h17?

Answer 1

What exactly is 3c7h17? Did you possibly mean 3 C₇H₁₇?

Not sure that C₇H₁₇ can exist because as written it is a hydrocarbon and so the number of hydrogens should be an even number, like C₇H₁₆ (heptane). But if you just want to use C₇H₁₇ then in every mole of this hypothetical molecule there are 17 moles of H atoms.

17 moles of H atoms x 6.02x10²³ atoms/mol = 1.02x10²⁵ H atoms

If the 3 in front of your compound is really meant to be there, then the number of H atoms would be 3 times this number.