1) mass = volume × density. Density of water equals 1 kg/L. That's why m = 1.4×1010 L × 1 kg/L = 1.4×1010 kg;
2) mass of F can be 1 kg per 1000000 kg of water, or 1 g per 1000000 g of water (thanks, there was a mistake in the previous comment);
3) to find the mass of F we need to find million part of water mass (million part of 1.4×1010). That's why we divide 1.4×1010 by 106 (1000000).
Thank you for your quick reply! I still have few questions. How did you get the mass of water is 1.4x10^10 kg? And why the mass of F is 1g per 1000000 kg of water? Isn't "1g of flourine per million GRAMS of water"?And why do you have to divided 100000 by 1.4x10^10?
1.Calculate the volume and mass of water needed to supply a city: 1.4×1010
2.Calculate the mass of fluorine (F) needed for preventing tooth decay: 1 kg per
1000000 kg of water, mass (F) = 1.4×1010/106 = 1.4×104 kg.
3. Calculatethe amount of NaF needed (NaF consists of 45% of F): 1.4×104/0.45 = 2.3×104
Still comfused!! would you mind explain?