The acid we used was 20g/dm3. This means that there were 20g of HCl for every 1 dm3 (a dm3 is 1000cm3, which means it's 1 litre). This is a measure of concentration.
If we dilute the acid, we can work out the new concentration. To do this, we must first work out how strong it is compared to the original solution. Do this by dividing the volume of acid in the solution by the the total volume. For example, if you used 10cm3 of acid, mixed with 10cm3 of water, that gives a total volume of 20cm3. Volume of acid / Total volume = 10 / 20 = 0.5
This tells us that the solution is half as strong as the full strength acid (this makes sense - half of the volume of the solution was acid).
As another example, if we used 5cm3 of acid and 10cm3 of water, that means 5cm3 (acid) / 15cm3 (total volume) = 0.333 or 1/3
Once we've calculated the ratio, then we can multiply this by the original concentration. Our original concentration was 20g/dm3, so in our first example: 0.5 x 20g/dm3 gives us a new concentration of 10g/dm3.
ALTERNATIVELY: concentration is also displayed like this: 0.5M [the capital M stands for moles per dm3, where a mole is the name given to a specific number of atoms or molecules]. If you prefer, you can use this as a value of concentration. The full strength acid is 0.5M, so in our first example, the solution would have a concentration of 0.5 x 0.5 = 0.25M
If you put a column in your results table showing the concentration of the acid solution, that'll make your results look a lot more professional.
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