Question

Many brands of toothpaste contain about 1.0 mg of fluoride per 1 g of toothpaste. The fluoride compound that is most often used in toothpaste is sodium fluoride (NaF) which is 45% fluoride by mass. How many milligrams of NaF are in a 200 g tube of toothpaste? Give your answer to 2 significant figures, but not in scientific notation and without units.

Answer 1

1.0mg per 1g

$\implies$ 0.001g per 1g of toothpaste

How many g of fluoride are in 200g of toothpaste

$=\dfrac{0.001gx200g}{1g}$ = 0.2g

Now, since this 0.2 g represents 45% by mass of fluoride in NaF

100% would represent?

$=\dfrac{0.2gx1}{0.45} = 0.4444g of NaF$

Changing that mass to milligrams gives;

0.4444g x 1000mg = 444.4mg

= 440mg (to 2sf)