Which of the following salts is the least soluble in water? a.) strontium phosphateb.) mercury bromide c.) thallium(i) sulfided.) silver bromidee.) copper iodidef.) silver arsenate

A. Strontium Phosphate 

Solubility product constant is an equilibrium constant for the maximum amount of solute that can dissolve to form an aqueous solution. The value of the constant describes a solution which is saturated. 

The greater the solubility product constant, the more soluble a solute is in the liquid. Consequently, the smaller the constant, the less soluble the solute is. The following list shows the solubility product constants of the given compounds in aqueous solutions at 298K in decreasing order:

Thallium Bromide - 3.71×10–6
Copper Iodide - 1.27×10–12
Silver Bromide - 5.35×10–13
Silver Arsenate - 1.03×10–22
Mercury Bromide - 6.40×10–23
Strontium Phosphate - 1×10–31

The salt with the smallest solubility product constant is strontium phosphate, and therefore is the one which is least soluble in water. 

Molarity is moles of solute over liters of solution. idk if that is what you are talking about, but you can convert moles of solute to grams of solute using dimensional analyzes by using molar mass found on the periodic table.