Posted by Annie on Monday, March 12, 2012 at 10:24am.
10% w/w means 10 g HCl in 100 g of solution.
10% w/v means 10 g HCl in 100 mL of solution.
Concd HCl has what mass HCl in 1000 mL? That is 1.19 g/mL x 1000 mL x 0.37 = 440.3g. That's 440.3 g HCl/1000 mL of soln. That is the same as 44.03 g HCl/100 mL of soln. We want 10 g/100 mL soln so how much soln should we take of this more concd one?
100 mL x 10/44.03 = 22.7 mL of the concd soln will provide the 10 g and you add water to make 100 mL final soln. That should be 10% w/v.
For the 10% w/w.
Concd HCl is 37% w/w; therefore, it contains 37 g HCl/100 g soln. You want 10 g so 100 x 10/37 = 27.02 g (notice that this is 27.02/1.19 = 22.7 mL from the above preparation). Then add enough water to make 100 g of soln (which will be 100-27.02 = 72.98g H2O). I don't know if this is acceptable or not since this will NOT be 100 mL but it will be close. I looked up the density of 10% HCl and it is 1.05 g/mL so 100 g will have a volume of about 95 mL. Since the problem did not give the density of the 10% soln, I assume this is what your prof wanted. If not you can correct it knowing the density of 10% HCl is 1.05 g/mL. I have used too many significant figures in the final numbers; you should correct that for your work.