Molarity

Suppose you add 10 grams of sodium chloride to 100 milliliters of water. Then, in a different container, you add 10 grams of table sugar to 100 milliliters of water. Do the two solutions contain the same number of solute particles? No, they do not, because the two different solutes have different molar masses.

To compare the number of solute particles in solutions, chemists often use moles to measure concentration. Recall that a mole is the amount of a substance that contains approximately 6.02 × 10²³ particles of that substance. Molarity is the number of moles of a solute dissolved per liter of solution. Use the following equation to calculate molarity.

$Molarity=\frac{Moles of solute}{Liters of solutions}$

To make a 1-molar (1M) solution of sodium chloride in water, first calculate the molar mass of the solute. Sodium chloride, NaCl, has a molar mass of 58.5 grams. If 58.5 grams of sodium chloride is mixed with enough water to make one liter of solution, the resulting solution is 1-molar.

Table sugar, C₁₂H₂₂O₁₁, has a molar mass of 342 grams. To make a 1-molar solution of table sugar in water, 342 grams of table sugar must be added to enough water to make one liter of solution.

Figure 13 A 1M NaCl solution contains 58.5 grams of NaCl per liter of solution. Calculating How much solute would you need to make to make one liter of a 0.2-molar solution of sodium chloride in water?