- To make a 925 sterling silver ring, a jeweller uses 15.73g of pure silver and 1.27g of copper. Calculate the percent composition of the solute in the alloy.
Some mixtures are not easy to recognise because we can't see where each substance is. If we mix sugar and water, we know the sugar is there because we taste it, but we can't see it. This is a homogeneous mixture.
In this example, we say that the sugar has dissolved in the water, which is why it is called a homogeneous mixture or solution. In a solution, the particles of all the substances are mixed together so well that it's impossible to distinguish them. All solutions have two components:
The solvent and the solute can be found in any state of aggregation. The solvent is most often a liquid, usually water, in which case we talk about an aqueous solution. Here are some examples:
| Solute | Solvent | Example |
|---|---|---|
| Solid | Liquid | Water with sugar |
| Liquid | Liquid | Water with alcohol |
| Gas | Liquid | Fizzy drinks |
Although, as we said, solvents are often liquids, there are also solvents that are not liquid (they can be solid or gas). For example:
To work with solutions, we need to know the proportion of the solute and solvent, that is, the concentration.
There are many ways to express a concentration but the easiest and most commonly-used one is the percent composition (by mass).
It's not necessary to work in grams. You just have to make sure to use the same units of mass in the numerator and denominator.
The result will not have units because it is a percentage.
First, we have to work out what the solute is and what the solvent is in the solution (alloy):
Next, we calculate the mass of the solution from the data:
m (solute) = 1.27g
m (solvent) = 15.73g
m (solution) = m (solute) + m (solvent) = 1.27g + 15.73g = 17g
Finally, we substitute our values in the equation for the percent composition of the solute:
Therefore, 925 Sterling Silver always contains 92.5% of pure silver and 7.5% of another metal, usually copper, as in this case.
Another common way to express a concentration relates the amount of solute to the volume of the solution.
Remember the relationship that exists between units of capacity and volume: if we make a cube of 1 dm and we fill it up to the top with liquid, the volume of the liquid in the cube is 1 L. So:
1 dm3 is equivalent to 1 L
As in the previous example exercise, we first have to identify the solute and the solvent:
Next, given the grams of the solute (15g), we have to calculate the volume of the solution in litres:
m (solute) = 15g
V (solution) = 250mL = 0.25L
Finally, we substitute these values into the mass concentration equation:
Therefore, the solution will have a concentration of 60g/L.
Physiological saline solution is made with sodium chloride (NaCl, otherwise known as salt) in water with 0.9% mass and is used a lot in hospitals. Here's how to prepare 100mL.
When we mix two substances to make a solution, the solute particles leave their original position and get distributed among the particles of the solvent; that way, the particles of the solute move in to occupy positions that were previously occupied by solvent particles.
Activity 15
Read the label on a bottle of mineral water. What solutes does it contain?
Activity 16
Name two non-liquid solutions that are not mentioned in your book.
Activity 17
Find out about the mixture of gases that make up natural gas.
Activity 18
Is the silver that jewellers use a pure substance or a mixture? What about 1, 2 and 5 cent coins? Use the Internet to find out.
Activity 19
Calculate the mass of acetic acid in 10 g of a commercial vinegar with a label indicating a percent composition of 6%.
Activity 20
Calculate the mass concentration in g/L of a solution that has 7 g of a pure substance in half a litre of water.
Activity 21
In how many litres of water would we have to dissolve 100 g of salt to obtain a solution with a mass concentration of 5 g/L?
Activity 22
What is the mass concentration of a solution with 12 g of potassium chloride and 300 cm3 of water?
Activity 23
What is the percent composition of sugar in water if it contains 30 g of solute in 600 g of water?
Activity 24
We know that the mass concentration of sodium chloride in a solution is 8 %. How many grams of sodium chloride is dissolved in 75 g of the solution?