Posted by Chris on .
In a lab we are doing, we need to find the theoreical amount of NaCl that we should have left after evaporating the water off an NaCl solution.
our molarity of the solution was .100 +/- .6%. We made a stock solution of .5836 g of NaCl/ 100 mL of water. We then measured out 10. +/- .02 mL of solution with a transfer pipet and evaporated the water.
Our calculation looks like this:
10. +/- .02 x (.100 (sub 0) mol +/- .6%/ 1L) x (1 L/1000 mL) x (58.44 g/mol) = .58 +/- .005 g.
Is this correct for calculating the theoretical amount of NaCl?
58.36 g/L of soln = 1M and that is that same as 0.5836 g/100 mL solution (you say 100 mL water but that isn't the same as 100 mL of solution).
If you take 1/10 of that (you took 10 mL and that is 1/10 of 100), then the aliquot you transferred to the evaporating dish must be 1/10 x 0.5836 = 0.05836. If your number of 0.58 g is correct, then you expect to find in the 10 mL what you started with in the 100 mL. Doesn't sound right to me. BUT I just ran through you arithmetic and obtained 0.058. Check your arithmetic. Check my thinking.
That does make sense, I was wondering why my calculations were yielding the same amount of NaCl with the 10. mL as the 100. mL. I might have made a calculation error. Thank you.