Aim: How can we use moles to express the concentrations of solutions?

Do Now:

1. Take out a calculator and reference tables.

2. How many moles are in 118.72L of H2O(g)?

3. How many grams are in 3.6mol of CaCO3?

Answers to do now

How can we determine the concentration of a solution?

• First a little vocabulary – There are two parts of a solution.

1. Solute – the part being dissolved.

2. Solvent – the part doing the dissolving.

• Concentration of a solution can be expressed in one of the following ways; molarity, percent by volume, percent by mass, or parts per million (ppm).

Determining molarity• Molarity is an expression of the number of

moles of a solute (salt) in 1L of solution. • The formula for molarity can be found on

reference table T.• The formula is:

• The unit of measurement for molarity is M. If the solution is 1M, that means there is 1mol (1gram formula mass) of salt in 1liter of solution.

Examples: • What is the molarity of a solution that contains

5mol NaCl, in 1L of solution?

• What is the molarity of a solution that contains 4.0 mol of NaOH in 0.50L of solution?

What if the example gives the grams of solute?

1. Calculate the gram formula mass.

2. Convert to moles.

3. Calculate molarity.

Example: What is the molarity of a solution containing 82.0g of Ca(NO3)2 in 1 liter of solution?

Another example: • What is the molarity of a KF(aq) solution

containing 116g of KF in 1.00L of solution?

What if we need to know how many liters of solution we have?

• Rearrange the formula so that it looks like this:

• Liters of solution =

• Example: What volume of a 2.0M solution is needed to provide 0.50mol of NaOH?

Moles of solutemolarity

Percent by Mass• The mass of an ingredient divided by the total

mass, expressed as a percent. This is essentially the same as a percent composition problem.

%mass =

Example: What is the percent mass of sodium hydroxide if 2.5g of NaOH are added to 50.00g of H2O?

Mass of the part Mass of the whole

X 100%

Percent by Volume• Used when two liquids are mixed to form a

solution. For example, if the label on a bottle of rubbing alcohol says 70% isopropyl alcohol, that means that 70% of the solution is isopropyl alcohol, and the rest is water.

• To calculate percent by volume, we use the following formula:

% by volume =

volume of solutevolume of solution

X 100%

An Example: • What is the percent by volume of alcohol if

50.0 mL of ethanol is diluted with water to form a total volume of 300.0 mL?

Parts Per Million (ppm)• A ratio between the mass of a solute and the total

mass of the solution. Useful for extremely dilute solutions when molarity and percent mass would be difficult to interpret. For example, only about 2g of Cl is used in a swimming pool. If molarity were to be calculated, it would be an extremely small number.

• ppm is calculated using the following formula:

• ppm =

grams of solutegrams of solution

X 1,000,000 ppm

Example: • Approximately 0.0043g of oxygen can be

dissolved in 100mL of water at 20°C. Express this in terms of ppm.

Making specific solutions• To make a solution with a specific molarity, we

must find the gram formula mass, and add it to 1liter of solvent (usually water).

• The following steps are used:

1.Determine the number of moles of solute needed by using the formula:

moles = MV = molarity x liters of solution

2. Convert moles to grams by finding the gram formula mass and then multiplying by the number of moles given.

Example: What mass of sodium carbonate is required to

prepare 2.00L of a 0.250M Na2CO3 solution?

Activity/Homework #

• Review Book Page 128. Copy and answer #24-33.