A.Mass of evaporating dish 21.345g B. Mass of evaporating dish and saturated NaCl solution 42.817g C. Mass of dish and NaCl after evaporation 38.915g

Question

Report the mass % of NaCl in this solution.  Write the numbers as a percentage with the correct number of significant figures as shown. (i.e. 25%) there should be no space between the numbers and the % sign.

Answer 1

To find the mass % of NaCl in solution, one needs to know both the mass of NaCl and the total mass of the solution.  Then mass of NaCl/mass of solution(x100%) = mass % NaCl.

 

mass of solution = 42.817 g - 21.345 g = 21.472 g

mass of NaCl = 38.915 g - 21.345 g = 17.57 g (see discussion below)

HOWEVER, if you subtract the mass of dish & NaCl from mass of dish & NaCl solution, you get 42.817 g - 38.915 g = 3.902 g NaCl

These values don't agree with values given in the problem.  Since the mass of the evaporating dish should remain constant, it makes sense that the mass of NaCl will be the difference between the mass before and after evaporation.  Thus, the mass of NaCl = 3.902 g

 

Mass % NaCl = 3.902 g/21.472 g (x100%) = 18.17% (to 4 sig figs b/c subtraction limits value to 3 decimal places and that is 4 sig. figs. and then division limits answer to 4 sig. figs)