Which solution contains 0.2 mol of potassium hydroxide, KOH? (multiple choice)

A. 1 cm^3 of 0.2 mol dm^3 KOH
B. 10 cm^3 of 0.2 mol dm^3 KOH
C. 100 cm^3 of 2.0 mol dm^3 KOH
D. 1 dm^3 of 2.0 mol dm^3 KOH

The answer is C. 100 cm^3 of 2.0 mol dm^3 KOH, why?

1 Answer
anor277
Feb 20, 2016

"Concentration" = "Number of moles (n)"/"Volume of solution (L)"

Explanation:

Thus, since "Concentration" = n/V, all we have to do is multiply the concentration by the volume to get the number of moles: we can even do so dimensionally, i.e. "Concentration "xx" Volume" = mol*cancel(L^-1)xxcancel(L) = mol as required.

0.100*Lxx2.0*mol*L^-1=0.200*mol.

Note that 1000 cm^3 = 1 L or 1 dm^3. If I expanded out dm^3, I would get (10^(-1)m)^3, = 10^(-3)m^3. Or 1000 L = 1 m^3.

You would not be asked to perform this conversion at A level; in 1st year chemistry I would expect it. You WOULD be expected to know that 1000 cm^-3 = 1 dm^3 = 1*"L". Use what unit you like, but be consistent. This is of course a lot of work to do for a single multiple choice question.

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