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I have the following question to solve:

Tungsten, $\ce{W}$, and chlorine, $\ce{Cl}$, form a series of compounds with the following compositions: \begin{array}{rr} \text{Mass % W} & \text{Mass % Cl}\\ \hline 72.17 & 27.83\\ 56.45 & 43.55\\ 50.91 & 49.09\\ 46.36 & 53.64\\ \end{array} If a molecule of each compound contains only one tungsten atom, what are the formulas for the four compounds?

My answer is as follows:

For one gram of tungsten, chlorine has mass $$ \begin{align} \frac{27.83}{72.17} &= 0.3302~\mathrm{g}\\[3pt] \frac{43.55}{56.45} &= 0.7715~\mathrm{g}\\[3pt] \frac{49.09}{50.91} &= 0.9643~\mathrm{g}\\[3pt] \frac{53.64}{46.36} &= 1.157~\mathrm{g}.\\ \end{align} $$

Since $\frac{0.7715}{0.3302}\approx\frac{7}{3}$, $\frac{0.9643}{0.3302}\approx3$, and $\frac{1.157}{0.3302}\approx\frac{7}{2}$, the number of atoms of chlorine for a given mass of tungsten are respectively in the ratio $6:14:18:21$.

So if a molecule of each compound contains only one tungsten atom, the formulas are $\ce{WCl_{6}}$, $\ce{WCl_{14}}$, $\ce{WCl_{18}}$, and $\ce{WCl_{21}}$.

Is this correct?

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    $\begingroup$ No. Look up "empirical formula" in a chemistry book or Google it. $\endgroup$
    – Janice DelMar
    May 13, 2012 at 15:49
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    $\begingroup$ This question served as a bad example for the homework policy. (It might be quite a good fit nowadays though.) $\endgroup$
    – Martin - マーチン ♦
    Feb 23, 2016 at 8:46

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We don't have a homework policy yet, but I'm quite sure that it will be against providing full answers.

Hints:

  • You're supposed to take into account molar masses. The ratios in a formula are molar ratios, so you need to use those, not mass ratios.

  • Read up a bit on emperical formulae, as Janice suggested.

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  • $\begingroup$ This answer is preventing the question from being deleted no? $\endgroup$
    – Ali Caglayan
    Jun 10, 2015 at 21:30
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    $\begingroup$ Just my opinion, the question should be kept since the homework policy uses it as an example of a bad question. That's especially true since the other two bad examples there have already been deleted. $\endgroup$
    – j_foster
    Sep 13, 2015 at 19:49
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    $\begingroup$ It seems that we may have become more lax on homework questions because if OP posted this question today, I think this would be considered pretty good. $\endgroup$
    – orthocresol
    Sep 30, 2015 at 18:25

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