I am preparing for A level chemistry exams. One of the questions I came across is

Calculate the $M_\mathrm r$ of unknown monoprotic acid. The volume is $15\,\rm cm^3$ and concentration $10.95\,\rm g\cdot dm^{-3}$.

I know how to calculate the number of moles and I know the equation to calculate $M_\mathrm r$ but could anyone give me some guidance on how to calculate the mass from this data?

  • $\begingroup$ @Ivan please note that units should be upright, not slanted. Thanks for the edits. :) $\endgroup$ – It's Over Nov 12 '15 at 7:27
  • $\begingroup$ Is it not just $10.95 \times 15/1000$? The concentration is in $\rm g\cdot dm^{-3}$. $\endgroup$ – surelyourejoking Nov 12 '15 at 8:13
  • $\begingroup$ @surelyourejoking You would, after that, have to divide by the number of moles. $\endgroup$ – orthocresol Nov 12 '15 at 8:58
  • $\begingroup$ @orthocresol Indeed. Is it possible to find that given the information? $\endgroup$ – surelyourejoking Nov 12 '15 at 8:59
  • $\begingroup$ @surelyourejoking No, not the information we are given, at least. But OP mentioned that they knew "how to calculate the number of miles", so I'm guessing there's an earlier part of the question that they hid from us. $\endgroup$ – orthocresol Nov 12 '15 at 9:02

The concentration of your acid is given as a mass concentration.

Without knowing the molar mass, the molar concentration and the amount of substance in a volume of $15~\mathrm{cm}^{-3}$ are not accessible.

However, knowing that

$\mathrm{1~L = 1~dm^{-3} = 1000~cm^{-3}}$

and therefore

$15~\mathrm{cm}^{-3} = 0.015~\mathrm{dm}^{-3}$

the mass of the acid (in $15~\mathrm{cm}^{-3}$) is given as

$m = 10.95~\mathrm{g\,dm^{−3}\cdot0.015~\mathrm{dm}^{-3} = 0.16425~g}$


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