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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