IB Chemistry: Solutions
- Distinguish between the terms solute, solvent, solution and concentration(g dm–3 and mol dm–3).
- Solute: The substance which has been dissolved.
- Solvent: The substance the solute was dissolved in. E.g. Water
- Solution: This is a combination of both the solute and the solvent.
- Concentration: This is a unit measured by the abundance of the constituent divided by the volume of the mixture (solution). In this case, the units of concentration will be gdm-3 or if we concert to moles, moldm-3.
- Solve problems involving concentration, amount of solute and volume of solution.
Before we learn how to solve problems involving concentration, amount of solute and volume of solution, we should first learn an equation:
We can obviously use algebra to rearrange this equation to :
- Which equation you arrange to depends on the context on the question. If you are asked to calculate the concentration, then use equation (1). If you are asked to calculate the volume, use equation (3)!
- NOTE: Sometimes, the examiners try to trick you by using the unit cm3 for volume. This is obviously wrong as the conventional unit we use is dm3. Therefore, you will have to divide the value in cm3 by 1000 to obtain the value in dm3, the desired unit. E.g. 50 cm3 will become 0.05 dm3, 1000 cm3 will become 1 dm3, etc.
Examples:
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- 8.5 g of NaNO3 (Mr=85) is dissolved in water to create 400 cm3 of solution. What is the concentration of the solution?
- Since we want to find concentration, we will use equation (1).
Let’s first find the Number of Moles.
As we learned earlier on in Unit 1, number of moles can be calculated simply by the equation:
- We know that the Molar Mass is 8.5 g, and the Molecular mass (Mr) = 85. If you have no idea what I’m talking about, please review 1.1-1.3 before coming back to this topic.
Therefore, if we plug in the numbers:
Now we know the number of moles of solutes, and we are given that the volume is 400 cm3, we can continue right?
Wrong.
Remember what I mentioned earlier? The units for volume has to be dm3, so we will have to divide 400 by 1000 to convert the units to dm3. (400 cm3/1000) = 0.4 dm3.
Ok, now we can continue.
Dilution of Solutions
Sometimes when a solution is diluted, the number of moles remains the same (kept constant) before and after the reaction, but the volume changes.
Here is an equation connecting volume and concentration provided the number of moles is a constant:
C1 = Initial concentration
V1 = Initial volume
C2 = Final Concentration
V2= Final volume
Let’s do a problem:
