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?

  • 2
    $\begingroup$ No. Look up "empirical formula" in a chemistry book or Google it. $\endgroup$ May 13, 2012 at 15:49
  • 1
    $\begingroup$ This question served as a bad example for the homework policy. (It might be quite a good fit nowadays though.) $\endgroup$ Feb 23, 2016 at 8:46

1 Answer 1


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


  • 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.

  • $\begingroup$ This answer is preventing the question from being deleted no? $\endgroup$ Jun 10, 2015 at 21:30
  • 1
    $\begingroup$ 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
  • 5
    $\begingroup$ 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. $\endgroup$ Sep 30, 2015 at 18:25

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.