Question

A particular brand of toothpaste claims to contain 0.24 wt% sodium fluoride. What mass in grams of fluoride ion would be present in the average amount of toothpaste used to brush one's teeth, about two grams?

Now this is a weight percent concentration question, I know that. wt% = mass of solute/mass of solution x 100. I've already gathered that sodium fluoride is my solute so I had done the math to get the mass of 41. 9882 g/mol which just turns into grams.

But I am worried about finding mass of solution. I was just thinking that since I didn't find any mass given to me in the problem that I'd put down a number like 100 g.

So would I have to go through the problem again of 41.99 (rounded up from 41.9882 or should I round up more?)/100g x 100 and now that I'm writing that, it seems rather stupid.

But am I on the right track? My professor taught us a lot of things but word problems like these he never seems to teach, it's always the straight forward ones just asking for wt% and give you the simple ones.

Answer 1

You're right about [(g solute/g solution)]*100 = Wt %. There are a number of points to clear up with what you've written.
1. No, I don't think you assume 100 g for the mass of the solution. The problems tells you in the last sentence the the mass of the solution is about 2 grams.

2. So (x/2)_*100 = 0.24%
Solve for x and that gives you g NaF.

3. The problem asks for g F^- and not grams NaF. Convert NaF to F by g NaF x (atomic mass F/molar mass NaF)

4. You have far too many numbers in your answers. The 0.24 determines how many s.f. you can have and that is 2 so I would round that 41.9882 to 42. Frankly with the mass solution being "about 2 grams" you know the precision/accuracy is not too good.