4. True or false: You should use Avogadro's number to convert from moles to molecules

True, to convert from moles to atoms, multiply the molar amount by Avogadro's number. To convert from atoms to moles, divide the atom amount by Avogadro's number (or multiply by its reciprocal).

At 70∘C , ammonium chloride has a solubility of about 62 g / 100 g H2O . This tells you that can only hope to dissolve 62 g of ammonium chloride for every 100 g of water before the solution becomes saturated.