# Difference between two moles in a reacion

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Hi there

As a Chemistry student I've never understood this simple idea and that's why I'm asking anyone to help knock some sense into me. So my problem is NOT how to calculate moles, my problem is understanding the difference between the moles that you calculate in a reaction, given the mass of the reactant or product and obviously calculating the relative atomic/molecular mass, and the moles that's given in a balanced reaction.  To make things a bit clearer, the calculated moles of a reaction is very small, small than zero in some cases followed by many other numbers before the first significant figure, but the moles presented in a balanced reaction are usually integers.

If anyone needs me to be more specific by all means ask and I will gladly provide an example as to what I'm stuck with.

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To make things a bit clearer, the calculated moles of a reaction is very small, small than zero in some cases followed by many other numbers before the first significant figure, but the moles presented in a balanced reaction are usually integers.

Can you provide an example?

A negative number of moles would be very weird.

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Posted (edited)

Hi there

As a Chemistry student I've never understood this simple idea and that's why I'm asking anyone to help knock some sense into me. So my problem is NOT how to calculate moles, my problem is understanding the difference between the moles that you calculate in a reaction, given the mass of the reactant or product and obviously calculating the relative atomic/molecular mass, and the moles that's given in a balanced reaction.  To make things a bit clearer, the calculated moles of a reaction is very small, small than zero in some cases followed by many other numbers before the first significant figure, but the moles presented in a balanced reaction are usually integers.

If anyone needs me to be more specific by all means ask and I will gladly provide an example as to what I'm stuck with.

If you think about it, a mole of many compounds is quite a large amount. For example a mole of table salt (NaCl) is roughly 23 +35 = 58g. So if you work with dilute solutions of things (especially things like strong acids which are dangerous to handle when concentrated), you are likely to be dealing with fractions of a mole in most cases. Whereas obviously the proportions given in a reaction scheme tell you what happens in whole numbers of moles, so as to give you the appropriate ratios to apply to smaller quantities.

So if you have HCl +NaOH -> NaCl + H2O, you can see that 1 mole of HCl reacts with 1 mole of NaOH to give 1 mole of NaCl and 1 mole of water, but if you only have 0.01 moles of HCl, then that will be enough to react with 0.01moles of NaOH and give you 0.01 moles of the products. So you just scale it accordingly.

I don't think you will ever come across -ve numbers of moles, though you may come across fractions of a mole expressed in standard form, e.g. 5 x 10⁻² moles for  0.05 moles.

Edited by exchemist
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small than zero in some cases followed by many other numbers before the first significant figure, but the moles presented in a balanced reaction are usually integers.

I wonder if you are getting confused about the difference between 1 mole and 0.001 moles per litre ?

The first is an absolute quantity ; the second is a concentration.

We try to work in integers (just as you say) when considering chemical reactions mathematically.

4Fe + 3O2 = 2Fe2O3

So 4 moles of iron reacts with 3 moles of oxygen to produce 2 moles of iron oxide.

But the atomic mass of iron is 56 grammes per mole.

So 4 moles is 4 * 56 = 224 grammes, which is a lot of iron to be putting into a test tube or reaction flask.

We are more likely to react one tenth of this or  4 * 5.6 grammes (= 22.4grammes = 0.4 mole) with 0.3 moles of oxygen.

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If anyone needs me to be more specific by all means ask and I will gladly provide an example as to what I'm stuck with.

I would really like to see an example to see what is confusing you.

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