This page looks at the various ways concentrations of solutions can be expressed and how you convert between them. It then looks at some simple calculations involving solutions. I am assuming you have already read and understood the pages about moles, calculations from equations involving masses and calculations involving gases. This page follows on from those.
There are two ways of describing concentration that you will have to be familiar with, as well as convert between them.
The cubic decimetre (dm There are 1000 cm | |

Note: A decimetre is a tenth of a meter - 10 cm. A cubic decimetre is the volume of a cube 10 cm x 10 cm x 10 cm - 1000 cm^{3}. If you have read the page about gas calculations you will already know that. | |

You could also write g dm So the negative sign in g dm Personally, although I would always write g dm
But the more common measure of concentration you will come across is moles per cubic decimetre, mol dm So you will come across expressions such as 0.10 mol dm And one further useful term - ^{-3}. It is given the symbol M.So our hydrochloric acid of concentration 0.10 mol dm
There is absolutely nothing new in this. You already know how to convert from grams to moles and vice versa. The fact that the number of grams or moles is present in a particular volume of solution makes no difference to that.
What is the concentration in moles per cubic decimetre of sodium hydroxide containing 20.0 g dm
What is the concentration in grams per cubic decimetre of sodium chloride solution containing 0.240 mol dm
You mustn't quote that to more than 3 significant figures - that' the accuracy used in the numbers you are working with.
For the moment we are only looking at examples involving just one solution. On the page about titration calculations, we will look at what happens if you have two solutions.
What mass of calcium carbonate, CaCO
With these calculations, always start with what you know most about. In this case you know both the volume and concentration of the acid. Work out how many moles of it you have got.
Once you start these calculations there is an inevitability about them. If you work out the number of moles of HCl, it is fairly obvious that you can use the equation to work out the number of moles of calcium carbonate. And it is also fairly obvious that once you have done that, you can work out the mass in grams of calcium carbonate. However complicated the question may look, if you start with what you know most about and work out the number of moles of it, everything else will follow.
I am going to use the same reaction because the numbers work out easily, but add a complication by referring to the volume of carbon dioxide given off. You won't be able to do this unless you have read the page about calculations involving gases. What volume of carbon dioxide, CO
Again the thing you know everything essential about is the hydrochloric acid. So start there and see where it leads you.
25.0 cm
Start with what you know most about and work out how many moles of it there are.
Don't quote your answer to more than 3 significant figures - that's the accuracy of the figures in the question. **Where would you like to go now?**-
**To the calculations menu . . .**
© Jim Clark 2021 |