1
$\begingroup$

How do I calculate the number of hydrogen atoms in $\pu{0.5 mol}$ of hydrogen gas? I presume the answer would probably somehow use the equation $$n = \frac{m}{M},$$ where $n$ is the amount of substance, $m$ is the mass, and $M$ is the molar mass. However, I'm not sure how to calculate it.

$\endgroup$
4
$\begingroup$

There is no need to use $$n = \frac{m}{M},$$ since you already have been given the amount of substance. You would need to use the equation, if you were given the mass $m$, e.g. as $\pu{0.5 mg}$.

If you want to calculate the number of hydrogen atoms in $\pu{0.5 mol}$ hydrogen gas, then you should consider that they are diatomic molecules, $\ce{H2}$. The unit mole is represented by the Avogadro constant and is $$N_\mathrm{A} = \pu{6.02214076×10^23 mol−1} \approx \pu{6.022×10^23 mol−1}$$ Each hydrogen molecule has two hydrogen atoms, therefore \begin{align} n(\ce{H}) &= 2 \cdot n(\ce{H2}),\\ N(\ce{H}) &= n(\ce{H}) \cdot N_\mathrm{A},\\ N(\ce{H}) &= 2 \cdot n(\ce{H2}) \cdot N_\mathrm{A},\\ N(\ce{H}) &= 2 \times \pu{0.5 mol} \times \pu{6.022×10^23 mol−1},\\ N(\ce{H}) &= \pu{6.022×10^23}. \end{align}

| improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.