Question

You need to determine how much (mass) of toothpaste to dilute to 50.00mL. Target concentration is 0.001M. The fluoride content of the toothpaste is 0.15g F- per 100mL toothpaste. The density is 1.4g/mL.

I don't know why I can't figure it out. Thank you for the help!

Answer 1

Concentration of fluoride = 0.15g/100mL = 0.15gm/19gmmol^-1/100ml = 7.89 *10^-4 moles/100mL

Molarity = 7.89 *10^-4 moles * 1000mL/100mL = 7.89 *10^-3 M

Final volume = 50mL, Final concentration 0.001M

M1V1 = M2V2 where M1 and V1 is the initial concentration and volume. M2 and V2 is the final concentration and volume.

V1 = M2V2/M1 = 50ml * 0.001M/ 7.89 *10^-3 M = 6.33mL

So, 6.33 ml has to be diluted to 50mL.

Density = mass/volume

mass of the toothpaste = density * volume = 1.4 g/mL * 6.33 mL = 8.86 gm