Help with chemistry question (1 Viewer)

[Bread_2_Toast]

I am having trouble getting the correct answer and cant work out where i am going wrong. Can anyone help?

Calculate the mass of cobalt (iii)flouride(molar mass = 115.93 g/mol) required to produce 65.00 ml of solution with a 0.2432 M concentration of flouride ions?

Step 1. - n = c*v
n = 0.2432 x 0.065
n = 0.0158
Step 2. COF3-
Step 3. ???m= n*m??
m= 0.0158 * 115.93
m = 1.831694 - X
Correct answer is 0.005269
Can't work out this last step of the equation.

 

[greenie1113]

The question is a trick question in that it gives you the concentration of the solution in terms of a ion inside it, instead of the solution as a whole.

Co3+ + 3F-
m= ?
M= 115.93
V= 0.065L
c= 0.2432

n=cv
= 0.2432 x 0.065
= 0.015808 of F3 ions.
Divide this by 3 to get the amount for one F ion, and you end up with the number you expected: 0.005269333. This is too many significant figures, of course, but it's a good habit to not drop figures mid-calculation, and even to keep things in memory or make the calculator do the entire thing at once, so that you get a more accurate result.

After this, multiply by the molar mass to get the expected mass of substance.
m=nM
= 0.005269333 x 115.93
= 0.610873813
= 0.61 g