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Concentration Packet

You have learned about the process of solvation and the factors that affect solubility. The concentration of a solution is a measure of how much solute is dissolved in a specific amount of solvent or solution. How would you describe the concentration of the solutions in the picture? Concentration may be described qualitatively using the words concentrated or dilute. In general a concentrated solution, as shown on the left in the picture, contains a large amount of solute. Conversely, a dilute solution contains a small amount of solute. How do you know that the tea on the right in the picture is a more dilute solution than the tea on the left?

• Concentration = amount of solute in a solution in relation to a particular amount of solvent

• Dilute solution = weak solution, contains a comparatively low amount of solute

• Concentrated solution = strong solution, contains a comparatively high amount of solute

In chemistry, we need more than a qualitative way to describe concentration. We need a way to specify, numerically what the concentration is. Most solutions are unsaturated, so we need a way to express the amount of solute and solvent they contain. For our course, we are going to have two ways to specify concentration: molarity and molality. Both base concentration on the moles of solute in a given amount of water.

| |Molarity |

|definition |number of moles of solute dissolved per liter of solution |

|abbreviation |M |

|formula | |

Molarity

General Relationship

|Here is the general relationship that you will be using over and over |molarity = |

|again. The molarity is equal to the number of moles of solute divided by|moles of solute |

|the volume of the solution measured in liters. If you like to think of |liter of solution |

|numbers and units instead of quantities look at the second version of | |

|the equation. | |

| | |

| |M = |

| |n moles |

| | v L |

| | |

| | |

| | |

| |2 M = |

| |6 moles |

| | 3 L |

| | |

| | |

| | |

Calculating Molarity from Moles and Volume

|Here we are given something to figure out. To get the molarity we need|What is the molarity of a solution containing 0.32 moles of NaCl in 3.4 |

|to divide the number of moles of NaCl by the volume of the solution. In|liters? |

|this case that is 0.32 moles NaCl divided by 3.4 L, and that gives | |

|0.094 M NaCl. |molarity = |

| |0.32 moles NaCl |

| | 3.4 L |

| | |

| |= |

| |0.094 M NaCl |

| | |

| | |

| | |

|Calculating Molarity from Mass and Volume |

|Calculating Molarity from Mass and Volume This one is a bit more |What is the molarity of a solution made by dissolving 2.5 g of NaCl in |

|difficult. To get molarity we still need to divide moles of solute by |enough water to make 125 ml of solution? |

|volume of solution. But this time we're not given the moles of solute. | |

|We have to calculate it from the mass of NaCl. |molarity = |

|We multiply 2.5 g NaCl by the conversion factor of 1 mole NaCl over the|moles of solute |

|formula weight of NaCl, 58.5 g. That tells us that we have 0.0427 mole |liter of solution |

|of NaCl. | |

|Now that we know the moles we can calculate the molarity. Moles of | |

|solute (0.0427) divided by the volume of the solution (0.125 L) gives | |

|us 0.34 M NaCl. |2.5 g NaCl x |

| |1 mole NaCl |

| |58.5 g NaCl |

| |= 0.0427 mole |

| | |

| | |

| | |

| |molarity = |

| |0.0427 mole NaCl |

| | 0.125 L |

| | |

| |= |

| |0.34 M NaCl |

| | |

| | |

| | |

For example:

[pic]

Molarity Problems

1. What is the molarity of an aqueous solution containing 40.0 g of glucose (C6H12O6) in 1.5 L of solution?

2. What is the molarity of a bleach solution containing 9.5 g of NaOCl per liter of bleach?

3. Calculate the molarity of 1.60 L of a solution containing 1.55 g of dissolved KBr.

[pic]Molarity Practice Problems

1. What is the molarity of the solution produced when 145 g of sodium chloride is dissolved in sufficient water to prepare 2.75 L of solution?

2. How many grams of potassium chloride are needed to prepare 0.750 L of a 1.50 M solution of potassium chloride in water?

3. What is the molarity of the solution produced when 85.6 g of hydrochloric acid is dissolved in sufficient water to prepare 0.385 L of solution?

4. To produce 3.00 L of a 1.90 M solution of sodium hydroxide, how many grams of sodium hydroxide must be dissolved?

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moles of solute

liter of solution

M =

How do you calculate moles? The first step to finding the amount of moles of substance is to determine the molar mass of the chemical formula. Molar mass is often expressed as g/mol. You can calculate molar mass by finding the number of atoms in the molecular formula, then adding the atomic mass of each element together to find the molecular weight.