Why is it to get from x amu---->xgram you need a mole of the substance.

I mean I underdstand this to be the case for Hydrogen. Why does a mole of a substance CONVERT the substances amu into grams?

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    $\begingroup$ Well, the moles of all elements have different weights too, so what's the problem? $\endgroup$ – Ivan Neretin Oct 5 '18 at 12:36
  • $\begingroup$ How does a mole of a substance equal its amu in grams? $\endgroup$ – user57928 Oct 5 '18 at 15:55
  • $\begingroup$ I cant see the answer on that lini. $\endgroup$ – user57928 Oct 5 '18 at 16:28

I often find units help me understand things like this. So, if:

1 g = mass of Avogadro's number of hydrogen atoms / protons / neutrons, and

1 amu = mass of a hydrogen atom / proton / neutron, and

1 mole = Avogadro's number of atoms or molecules of a substance, then:

if I make a substance heavier than hydrogen, the atomic or molecular mass will go up by one amu for every proton or neutron, and the mass of a mole of that substance will go up by 1 gram for each of those extra protons or neutrons, since a mole of protons or neutrons has a mass of 1 g.

  • $\begingroup$ Why with a different amu its still 1 mole in grams? $\endgroup$ – user57928 Oct 5 '18 at 16:29

Both AMU and grams are units for expressing mass. AMU is used for weighing single protons, neutrons ore atoms while grams are used for more convenient amounts. So: 1 AMU = mass of one proton = mass of one H-atom (neglecting the electron) = 1,7*10^-24 g. But if you want to do some calculations in gram, that figure is rather unhandy, and instead you use the mass of one mole. One mole is, as you probably know, 6.23*10^23 pieces of whatever we are talking about (Avogados number). The mass of one mole of H is therefore: 1 AMU * 1 mole = 1 gram (round figures). But as Ivan said: other elements weights more. An example: carbon (6 protons + 6neutrons) weigh 12 AMU and one mole (6.23*10^23 C-atoms) weighs: 12 AMU * 1 mole = 12 grams. Ore to be more precise: 12 grams per mole

  • $\begingroup$ So can we say that 1 mole =g/amu $\endgroup$ – user57928 Oct 5 '18 at 16:32
  • $\begingroup$ Fundamentally yes, but what is your point with this? One mole is a fixed number You can look up, so there is no reason for calculating it? $\endgroup$ – FrankS Oct 8 '18 at 14:41

There are two different ways to measure a quantity. Think about bananas. You can weigh the bananas or you can count them.

Now let's think about a reaction

$$\ce{2Na + Cl_2 -> 2NaCl}$$

It takes one atom of sodium to react with one atom of chlorine to get one molecule of sodium chloride. Now we can't really count individual atoms in a typical chem lab, so we need some way to covert between a count of atoms and a mass of atoms. The way to convert is through the use of "moles".

A mole is just a big counting number like a dozen or a million. But a mole is a much much bigger number. The value is about $6.022\times10^{23}$. So if you "count out" $6.022\times10^{23}$ atoms of sodium you get a mass 22.99 grams of sodium atoms that corresponds to the atomic mass of sodium.

The same notion applies to the chlorine gas. $6.022\times10^{23}$ molecules of chlorine gas have a mass of 70.90 grams of chlorine gas. But each chlorine molecule has two atoms of chlorine. So we'd only need 35.45 grams of chlorine gas to react with 22.99 grams of sodium.

  • $\begingroup$ You say "if you "count out" atoms of sodium you get a mass 22.99 grams of sodium" , how do you know that ? $\endgroup$ – user57928 Oct 5 '18 at 16:31
  • $\begingroup$ Better example than bananas would perhaps be water. A gallon of water weighs 8.34 pounds. So I can measure water in pounds or gallons. If I measure gallons, I can convert to pounds by knowing the conversion constant. The primary units of measure form the International System of Units. $\endgroup$ – MaxW Oct 5 '18 at 18:21

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