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.
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.
You're on the right track with your understanding of weight percent concentration and how to calculate it.
To find the mass of fluoride ion in the toothpaste, you first need to determine the mass of the solution (toothpaste) used. In this case, you mentioned that the average amount of toothpaste used for brushing is two grams. Therefore, the mass of the solution is two grams.
Next, you need to calculate the mass of sodium fluoride (solute) present in the toothpaste. You correctly stated that the molar mass of sodium fluoride is approximately 41.99 grams/mol. However, you don't need to calculate it further at this point.
To calculate the mass of fluoride ion, you can use the weight percent concentration formula you mentioned correctly: wt% = mass of solute / mass of solution x 100.
Plugging in the numbers, we have: 0.24 wt% = mass of sodium fluoride / 2 g x 100.
Now, let's rearrange the equation to solve for the mass of sodium fluoride:
mass of sodium fluoride = (0.24 wt% / 100) x 2 g.
Calculating this expression, we get: mass of sodium fluoride = (0.0024) x 2 g = 0.0048 g.
Therefore, the mass of fluoride ion present in the average amount of toothpaste used (two grams) is approximately 0.0048 grams.
Remember to always pay attention to the units in calculations to make sure they are consistent and to round your final answer to an appropriate number of significant figures.