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Hydralazine is a drug used to treat heart disease. It is $59.99\%$ carbon, $5.03\%$ hydrogen, and $34.98\%$ nitrogen and has a molecular mass of $160.178$.

What is the molecular formula for hydralazine?

I did: \begin{align} 59.99~\ce{C}/12.011&=4.994\\ 5.03~\ce{H}/1.01&=4.9801\\ 34.98~\ce{N}/14.007&=2.4973. \end{align}

Then divided all $3$ by $2.4973$, and got 2 carbon, 2 hydrogen, and 1 nitrogen atoms i.e. $\ce{C_2H_2N}$. The online homework said that the stoichiometry was wrong. I don't know what I did wrong.

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    $\begingroup$ Can you show your calculations that led you to that answer? That way someone can show you where you went wrong. $\endgroup$
    – jonsca
    Jun 23, 2014 at 0:50
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    $\begingroup$ Good, but does the molecular weight of $\ce{C2H2N}$ add up to 160.178? If not, what do you need to multiply by to get there? $\endgroup$
    – ron
    Jun 23, 2014 at 1:12

1 Answer 1

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The percentages are based on the molar mass. You have to multiply the percentages with the molar mass and divide by the mass of the atoms.

\begin{align} \ce{C}:&& \frac{0.5999 \cdot 160.178}{12.011} &\approx 8\\ \ce{H}:&& \frac{0.0503 \cdot 160.178}{1.01} &\approx 8\\ \ce{N}:&& \frac{0.3498 \cdot 160.178}{14.007} &\approx 4\\ && \implies \ce{C8H8N4}\\ \end{align}

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