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High School Chemistry: Help and Review40 chapters | 292 lessons | 1 flashcard set

Instructor:
*Amanda Robb*

In this practical application lesson, you'll be learning how to calculate different measures of solution concentrations including the molarity, mole fraction, molality, mass percent, and volume percent of solutions.

Even though solution concentrations might sound a bit intimidating, chances are you're already quite familiar with solutions. A **solution** is any homogenous mixture of two or more substances. The substance of greater quantity is called the **solvent** and the substance being dissolved is called the **solute**.

If you've ever made lemonade from a package, instant hot chocolate, or Gatorade from a mix, then you already know about solvents and solutes. The ratio of solute to solvent is the **concentration** of a solution. Today, we're going to look at examples of problems in which we calculate the concentration of solutions using different methods.

Calculate the molarity of a solution of two liters of water mixed with 30 grams of sodium chloride (NaCl).

To calculate molarity, we first have to recall that **molarity** is equal to the moles of solute per liter of solution.

First, make sure you have your solution volume in liters. This problem gives you the quantity in liters, but if you are given milliliters or gallons, you must convert first.

Next, calculate the number of moles in solution. To do this, you must find the molar mass of your compound. Find your nearest periodic table and then locate the molar mass of each element in the compound. For sodium chloride, we add the molar mass of sodium (22 g/mol) with the molar mass of chloride (35 g/mol) to get the total molar mass of 57 g/mol.

Next, set up a conversion to find out how many moles are in the total mass of solute you have. Flip your molar mass upside down so you have a fraction of 1 mole over the mass in grams. Then, multiply that by the number of grams total.

For our example we get:

30g NaCl x (1 mol / 57g) = 0.52 moles NaCl

Now, you can simply divide the number of moles by the liters of solution and get molarity.

0.52 moles NaCl / 2L water = 0.26M

Calculate the mole fraction of a solution of two liters of water mixed with 30g of sodium chloride.

Another way to calculate concentration is the mole fraction. Here, we calculate the ratio of the moles of solute to the moles of the entire solution.

First, calculate the moles of each substance in the solution. Here, we would do this just as we did to calculate molarity using the molar masses of sodium and chloride. The molar mass of 30g of sodium chloride is 57 g/mol, giving us 0.52 moles sodium chloride.

Similarly, for water, the periodic table can be used to find the molar mass (18 g/mol). Then, we can use the density of water to find the mass from the volume given, which is 1g/mL. So, we have 2,000g of water. Using our formula to calculate moles we get:

2,000g water / (1 mole / 18g) = 111.1 moles water

Next, add up all the moles to get total moles. For our example, 0.52 moles NaCl + 111.1 moles water = 111.62 moles total.

Now, divide the moles of solute by the total number of moles in solution to get the mole fraction.

0.52 moles NaCl / 111.62 moles total = 0.004

Molality might sound a lot like molarity, but it's actually different. **Molality** is the moles of solvent divided by the mass of the solvent in kilograms. Let's calculate molality for this example.

Again, use the molar mass and then convert the mass given in a problem to moles. For our example, we have 0.52 moles of sodium chloride.

Again, we can convert a volume of water to mass by using the density of water, 1g/mL. Since we have 2,000 mL of water, this is equal to 2,000g of water, or 2kg.

Now, divide your moles of solute by the kilograms of solvent. In our example we get:

0.52 moles NaCl / 2kg water = 0.26m

Now, let's calculate the mass percent of 30g of NaCl in 2L of water. **Mass percent** is the mass of the solute divided by the mass of the solution.

Since this problem already tells you the mass of NaCl, you don't need to do the calculations. But, if the problem gave you the moles, you could work backward using the molar mass as explained in the first example problem.

Now, we will use the density of water to calculate its mass as we've done in previous problems. The density of water is 1g/mL, so for 2L of water, or 2000mL of water, the density is 2000g. To get the mass of the solution, add up the mass of the solvent and solute. For our example, this is 2000g of water + 30g of NaCl = 2030g total.

Now, divide the mass of the solute by the mass of the solution. Here, we get 30g NaCl / 2030g total = 0.014 or 1.4%.

**Volume percent** is used when both solute and solvent are liquids. Let's say you want to dissolve 30mL of ethanol in 1000mL of water. To solve this type of problem, simply divide the volume of solute (ethanol) by the volume of solution.

In our example, we combined 35mL of ethanol with 1000mL of water, so the total volume of solution is 1035mL.

So, 35mL / 1035mL = 0.034 or 3.4% ethanol by volume.

**Solutions** are homogenous mixtures made of **solutes** dissolved in **solvents**. The ratio of solute to solvent is the **concentration** and can be calculated in several ways. **Molarity** is the moles per liter solvent. **Mole fraction** is the moles of solute per total moles of solution. **Molality** is the moles of solute per kilogram of solvent. **Percent mass** is calculated by dividing the mass of solute by the total mass of solution. The **volume percent** is used when two liquids are combined and is the volume of solute divided by the volume of solution.

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High School Chemistry: Help and Review40 chapters | 292 lessons | 1 flashcard set

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