Calculating the number of neutrons in a sample


> 
> Which of the following contains one mole of neutrons?
> 
> 
> A. $\pu{1 g}$ of $\ce{^1\_1H}$  
> 
> B. $\pu{1 g}$ of $\ce{^{12}\_6C}$  
> 
> C. $\pu{2 g}$ of $\ce{^{24}\_{12}Mg}$  
> 
> D. $\pu{2 g}$ of $\ce{^{23}\_{10}Ne}$
> 
> 
> 


Here's a question which I don't know how to answer. I tried using the equation:


moles=mass/mr


but that did not match the answer given, which is (C). I keep doing 2/24 which doesn't appear to equal 1 mole.




The formula


$$\text{amount of substance} = \frac{\text{mass}}{\text{molar mass}}$$


tells you that you have 1/12 moles of *magnesium*, since you are taking the mass of magnesium (2 g) and dividing by the molecular mass of magnesium (24 g/mol). However, the question is not asking about the number of moles of *magnesium*.


Each magnesium atom has 24 - 12 = 12 neutrons. Consequently, 1/12 moles of magnesium has (1/12)(12) = 1 mole of neutrons.


 7

Answer A has no neutrons
Answer B has 1/12 mole of carbon with 6 moles of neutrons per mole. So there's only one-half mole of neutrons.
Answer C has 2/24 or 1/12 mole of Mg with 12 moles of neutrons (and 12 moles of protons) per mole of Mg. 12/12 = 1, the answer.
Answer D has 2/23 mole Ne with 23-10 moles of Neutrons per mole of Ne. So is more than 1 mole of neutrons in the answer.


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