Question

I'm having trouble understanding exactly what an "amu" (or "u" or "Da" as I believe are more modern labels). What is a very layman explanation for "amu?"

Answer 1

it is defined as 1/12 of a single carbon-12 atom, generally can be taken as the same as the mass of one proton or one neutron

Answer 2

"amu" is an acronym for "atomic mass unit", 1 amu is defined as 1/12 of the mass of C-12.

"Da" is a Dalton which means "g/mol", redundant but has been used historically for proteins

Answer 3

Da, as in the abbreviation of Dalton, is the same unit as amu, but more common in the biochem lingo

Answer 4

amu is the same as gram per mole is because number of atoms in 12 gram of C-12 is defined as one mole

Answer 5

So then how would grams be converted to moles to atoms, or some combination of the two? Is it similar to some three variable equation like d=rt?

Answer 6

\(\sf moles=\dfrac{mass}{Molar~mass}=\dfrac{\#~of~particles}{avogadro's~number }\)

here "particles" can be a number of different things (atoms, molecules) depending on the context.

Answer 7

@aaronq So for example, if I'm told to find the number of moles and atoms in 0.400g of Sodium, it would be\[\sf moles=\dfrac{mass}{Molar~mass}=\dfrac{0.400g~Na}{6.022x10^{23}}=6.64x10^{-25}\]
and for atoms it would be:
\[\sf atoms = moles * molar~mass = 6.64x10^{-25} * 6.022x10^{23} = 0.4~atoms\]

Is my process here correct?

Answer 8

You've made a mistake. You wrote 6.022*10^23 as the molar mass of Na which is incorrect.

Answer 9

@aaronq I thought Avogadro's number was supposed to be there as it's in the denominator of the last quotient in your equation. Is this correct instead?
\[\sf moles = \dfrac{mass}{Molar~mass}= \dfrac{0.400g Na}{22.99~amu} = 1.74*10^{-2}~moles\]
\[\sf atoms = moles * molar~mass = (1.74*10^{-2})*(6.022*10^{23})= 1.05*10^{22} ~atoms\]

Answer 10

Its the denominator when particles are involved - not mass, however.

What you wrote now is correct.

Answer 11

@aaronq So if I am given a substance with a mass in grams (kilograms or any mass related SI unit), and I need the moles of that substance, I convert it to grams, and divide it my it's atomic mass unit, and if I am given anything that is not grams or unable to be represented in grams (atoms, molecules, apples, trees), then I would use particles over Avogadro number?

Answer 12

And by "anything unable to be represented in grams" I'm thinking absolutely anything that can't be modeled as "200g Na" or more generically "(mass in grams) of (substance)."
Examples being: atoms, molecules, headphones, calculators, iPhones

Answer 13

yup, that is correct.