4
$\begingroup$

I have a question that is to find the concentration of magnesium ion in ppm for a $0.025\ \%$ solution of $\ce{MgCl2}$. I can use this formula: $$\text{concentration in ppm} = \frac{\text{mass solute}_\mathrm{(g)}}{\text{mass solution}_\mathrm{(g)}}\times10^6$$

Assuming $100\ \mathrm g$ of solution, $2.5 \mathrm g$ solute/$\ce{MgCl2}$, and $97.5\ \mathrm g$ solvent, I get $\pu{95.205u}$ for $\ce{MgCl2}$, and then $0.026259\ \mathrm{mol}$ of $\ce{MgCl2}$, and thus $0.026259\ \mathrm{mol}$ of $\ce{Mg}$. That times $24.305\ \mathrm{g\ mol}^{-1}$ for $ \ce{Mg}$ gives me $0.63823\ \mathrm g$ of $\ce{Mg}$. $\frac{0.63823\ \mathrm g}{100\ \mathrm g}$ solution gives me $0.00638$, so that multiplied by $10^6$ would be $6382.28$. But the only options for the questions don't go over $500$. Like $64\ \mathrm{ppm}$ or $500\ \mathrm{ppm}$. Nothing even close to as high as I'm getting.

$\endgroup$
6
$\begingroup$

You're using $2.5\ \mathrm g$ of solute out of $100\ \mathrm g$ of solution, which is $2.5\ \%$, but the initial question is asking about a $0.025\ \%$ solution, which is why your answer is two orders of magnitude too large. You should be using $0.025\ \mathrm g$ of solute and $99.975\ \mathrm g$ of solvent.

As a general helpful hint for answering multiple choice questions, when you notice that one of the answers has the same coefficient but is off by a couple orders of magnitude (in this case the $64\ \mathrm{ppm}$ answer) go and double check all your initial concentrations and volumes, you probably forgot to account for a "$\%$" or a "$\mathrm k$" somewhere.

$\endgroup$

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.