I'm currently editing some chemistry test questions, and I have several sentences like the following:

  • What is the total number of moles of HCl produced when 3 moles of hydrogen is completely consumed?
  • How many moles of hydrogen are consumed in this reaction?

I originally flagged "are" in the latter sentence to be changed to "is" in order to be consistent, but intuition is leading me to doubt this. Does the "How many..." construction of the sentence make "moles" dictate agreement in this case?

I suspect that "How many..." contextually sets the expectation that the subject is a count noun ("moles" in this case), whereas in cases like the first sentence, there is no clear contextual indication whether the subject is a count or mass noun. Does anyone know if there is a clear rule for cases like this? If not, would you stick with "are" in the second sentence or change it to "is"?

  • 1
    I think your intuition is perfectly correct: 1, "is" and 2 "are". Jun 14 '12 at 16:31
  • 2
    Another curiosity in the same vein: "2 moles of hydrogen is consumed" and its (putative) analysis "1.20 × 10^24 molecules of hydrogen are consumed" have different agreement.
    – Excellll
    Jun 14 '12 at 17:55

Moles are units, and English allows the use of 'is' with units.

Three hours is not enough to finish the task.

You have to 'are' when the units are referred to as individuals and not as a total amount.

The next three miles of the Paradise Trail are the most beautiful of the entire hike.

However, 'how many' requires an 'are', whereas 'how much' requires 'is'.

How many moles of hydrogen are consumed in this reaction?
How much hydrogen (in moles) is consumed in this reaction?

This is because 'how much' is always used with mass nouns and thus takes 'is', while 'how many' is always used with countable nouns and thus takes 'are'.

  • Have you noticed, though, that in some instances, the mass noun - countable noun distinction seems blurred? Thus how many potatoes (count noun) and how much mash (mass noun) are obvious, how much rice shows that we treat rice as a pseudo-mass noun, like confetti, but peas seem / peas seems to need special treatment. Jun 16 '12 at 21:26

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