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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.

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2 Answers 2

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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.

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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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