I'm a little bit confused here, does the mole of a solution equals to the mole of the solvent + mole of the solute?
Any help would be appreciated. Thank you.
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$\begingroup$ Welcome to Chemistry SE! What efforts have you made to find the answer on your own? could you add that in your question? $\endgroup$– Safdar FaisalCommented Jul 31, 2020 at 15:13
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2$\begingroup$ A mole of solution has no meaning. Moles are applied only to pure substances, not to mixtures or solutions. $\endgroup$– MauriceCommented Jul 31, 2020 at 16:31
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Maurice is wrong about moles only being applied to pure substances. Consider a flow of air that goes into a process. It is perfectly acceptable to say that there 10 moles of air per min in flow. Air is a mixture. It contains about 79% nitrogen gas and 21% oxygen gas. Knowing this, I can now tell you that because 10 moles of air are flowing, this also means that 7.9 moles of nitrogen are flowing and 2.1 moles of oxygen are flowing!
By this same logic say I have a solution of household white vinegar. It IS in fact a solution. Acetic acid is the solute and water is the solvent. Regular white vinegar usually has about 5% acetic acid by weight. So say I had 100 g of white vinegar. 5 g of that solution would be acetic acid and the other 95 g would be water.
So to calculate the average molar mass of a solution using mass fractions, the formula is 1/(avgMW) = sum(massfrac of component i/MW of component i)
So in the vinegar case above, the molecular weight of a 5% solution of household white vinegar would be 18.95 g/mol. I can now use this to tell you how many mols are in 100 g of 5% white vinegar (it's 5.28).
I hope this let's you see that moles are not additive in the manner that you described in your question. If you're still not understanding, think about the fact that in a chemical reaction, mols are not always conserved, but mass always is. Mols are really just a convenient stoichiometric tool. Like there are 12 eggs in a dozen, there are 18.02g in a mol of water.
Hope this helps :)
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1$\begingroup$ @ cqferrier. You are right in stating that one mole air has a meaning, because air has a constant composition. I am not so sure about the meaning of one mole vinegar. Vinegar has not always the same composition. 5% is an average value. Some vinegars may have up to 6% acetic acid, $\endgroup$– MauriceCommented Jul 31, 2020 at 19:09
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$\begingroup$ By other words, the particular composition of a mixture must be supposed. $\endgroup$– PoutnikCommented Jul 31, 2020 at 20:19
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$\begingroup$ A mole of vinegar solution has meaning if the concentration is given. However, it has no useful meaning, because that mole of solution is not an amount that is used for a stoichiometric relationship. $\endgroup$– ZheCommented Jul 31, 2020 at 20:23