How to Calculate Molality of a Solution;

Molality is used to express the concentration of a solution when you are performing experiments that involve temperature changes or are working with colligative properties. Note that with aqueous solutions at room temperature, the density of water is approximately 1 kg/L, so M and m are nearly the same.

Calculate Molality: moles solute per kilogram solvent;

symbol: m

m = moles / kilogram

Example: What is the molality of a solution of 3 grams of KCl (potassium chloride) in 250 ml of water?

First, determine how many moles are present in 3 grams of KCl. Start by looking up the number of grams per mole of potassium and chlorine on a periodic table. Then add them together to get the grams per mole for KCl.

K = 39.1 g/mol

Cl = 35.5 g/mol

KCl = 39.1 + 35.5 = 74.6 g/mol

For 3 grams of KCl, the number of moles is:

(1 mole / 74.6 g) * 3 grams = 3 / 74.6 = 0.040 moles

Express this as moles per kilogram solution. Now, you have 250 ml of water, which is about 250 g of water (assuming a density of 1 g/ml), but you also have 3 grams of solute, so the total mass of the solution is closer to 253 grams than 250. Using 2 significant figures, it’s the same thing. If you have more precise measurements, don’t forget to include the mass of solute in your calculation!

250 g = 0.25 kg

m = 0.040 moles / 0.25 kg = 0.16 m KCl (0.16 molal solution).

How to Calculate Volume Percent Concentration of a Solution;

Volume percent is the volume of solute per volume of solution. This unit is used when mixing together volumes of two solutions to prepare a new solution. When you mix solutions, the volumes aren’t always additive, so volume percent is a good way to express concentration. The solute is the liquid present in a smaller amount, while the solution is the liquid present in a larger amount.

Calculate Volume Percent: volume of solute per volume of solution (not volume of solvent), multiplied by 100%

symbol: v/v %

v/v % = liters/liters x 100% or milliliters/milliliters x 100% (doesn’t matter what units of volume you use as long as they are the same for solute and solution)

Example: What is the volume percent of ethanol if you dilute 5.0 milliliters of ethanol with water to obtain a 75-milliliter solution?

v/v % = 5.0 ml alcohol / 75 ml solution x 100% = 6.7% ethanol solution, by volume

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