Source: Coffee, Christy Jordan; Southern Plate

In our everyday lives, we describe the strength of solutions using qualitative terms such as “strong” or “weak.” Coffee is one example. Some people like strong, black coffee. Other people prefer to weaken coffee by adding milk or cream.

If you’ve ever made a drink from a powdered mix drink, then you are probably familiar with solution concentrations. If you don’t mix the powder with enough water, the solution will be too strong and will most likely not taste good. If you mix too much water, then the solution will be too weak and flavorless.

Chemists often use solutions of varying concentrations in lab work. The strength of a chemical solution affects how it reacts. Science depends on measurable quantities that can be replicated. Therefore, scientists use molarity as a measure of the strength of a solution. Molarity is the ratio of moles of solute to liters of solution. A strong solution would have a higher molarity than a weak one.

Source: Concentrations, Pascoes Science

Below is the molarity equation as it appears on the Chemistry STAAR Reference Materials.

In order to calculate the molarity of a solution, you will need to know the number of moles of solute and the total volume of the solution.

To Calculate Molarity

Source: Scale, JP7Numeracy Blogspot

1. Calculate the number of moles of solute present. Remember, you can’t mass in moles. Use the formula mass of the element/compound to convert between grams and moles. Refer to module 3, lessons 9 and 10 if you need additional help on mole conversions.
2. Calculate, or determine, the number of liters of solution present.
   

   Source: Beaker with solution, JP7Numeracy Blogspot

3. Divide the number of moles of solute by the number of liters of solution. The units for molarity are moles/liters, and are often abbreviated as mol/L or simply M.

Examine the two Material Safety Data Sheets (MSDS) below to see how changing the concentration of the solution changes its reactivity. In your notes, answer the questions that follow.

Questions

1. The two Material Safety Data Sheets are for what solution?

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   Sodium Hydroxide or NaOH

2. Which of the following solutions has the greatest moles of NaOH in 1L? Explain how you chose your answer.
   
   

   6M NaOH       3M NaOH      1M NaOH      0.5M NaOH

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   6M NaOH has the greatest moles because it has the highest molarity. 6M means it has 6 moles of NaOH in every 1 L of solution, which is greater than any of the other solutions listed.

In the next section, you will walk through how to use molarity to create a solution and how to solve the sample problems.