The concept of amount in moles is central to chemistry because it allows us to keep track of the number of molecules, atoms, or ions present in a given sample of a substance. Amount in moles is a counting unit (6.022 × 1023 mol–1). Other more familiar counting units are a pair (2 pr–1) and a dozen (12 dozen–1).
n = cVThe amount in moles of substance in a
The amounts in moles of any pair of substances in a balanced equation are related through the coefficients in the balanced equation for the reaction as shown below.
If n(reaction) is predicted using n(B)
n(A) consumed = a × n(reaction)
n(C) produced = b × n(reaction)
n(D) produced = d × n(reaction)The amount in moles of reaction is also a very useful concept.
It is necessary to do the division shown so that the amount in moles of reaction does not depend on the substance used to calculate it.
The amount in moles of other substances that are consumed or produced in the reaction can be calculated from the known amount in moles of reaction.
One mole of water molecules contains 6.022 × 1023 H2O. One mole of helium atoms contains 6.022 × 1023 He.
One mole of sodium ions contains 6.022 × 1023 Na+.
One mole of sodium ions contains 6.022 × 1023 Na+.
n = cV
weighed sample can be calculated by dividing the mass (m) by the molar mass (M), the mass of 1 mole of substance
solution of known concentration (c) in ##REGAVAR(ConcenUnit)## and volume (V) in can be calculated by multiplying these.
Rearranged forms of these relationships can be used to calculate any of the variables in these equations provided the other two are known (m = nM and c=n/V)
solution of known concentration (c) in ##REGAVAR(ConcenUnit)## and volume (V) in can be calculated by multiplying these.
Rearranged forms of these relationships can be used to calculate any of the variables in these equations provided the other two are known (m = nM and c=n/V)
The amounts in moles of any pair of substances in a balanced equation are related through the coefficients in the balanced equation for the reaction as shown below.
For a reaction aA + bB 
cC + dD
where A,B,C and D are substances, and a, b, c and d are the stoichiometric coefficients
cC + dD| n(A) | = | n(B) |
| a | b |
where A,B,C and D are substances, and a, b, c and d are the stoichiometric coefficients
| n(reaction) = | n(A) | = | n(B) | = | n(C) | = | n(D) |
| a | b | c | d |
If n(reaction) is predicted using n(B)
n(A) consumed = a × n(reaction)
n(C) produced = b × n(reaction)
n(D) produced = d × n(reaction)
As shown at the right, the amount in moles of reaction that occurs for a given quantity of substance depends on the amount in moles of the substance and on the coefficient on the substance in the balanced equation.
It is necessary to do the division shown so that the amount in moles of reaction does not depend on the substance used to calculate it.
The amount in moles of other substances that are consumed or produced in the reaction can be calculated from the known amount in moles of reaction.
The two-step process of calculating the amount in moles of substance A that is consumed by substance B using amount in moles of reaction is equivalent to using n(A)/a = n(B)/b; however the two step process involving first a simple division and then a simple multiplication may be simpler for beginning students than the "cross-multiplication" involved in using n(A)/a=n(B)/b. Furthermore the concept of amount in moles of reaction is important in more advanced problems involving calculating yields.