A chemist requires 0.854 mol Na2CO3 for a reaction. How many grams does this correspond to?

Question

A chemist requires 0.854 mol Na2CO3 for a reaction. How many grams does this correspond to?

Answer 1

See below asterisk-bound section for calculation requested. Answer at very bottom.

You'll have to either sit through explanation, or scroll past it.

In essence, you are finding molar mass of the salt composed of molar masses of its constituent atoms...

then multiplying the number of moles you have by that molar mass (the grams per mole) of our salt...

so that moles of salt times (grams per mole of salt) will equal "grams of salt," okay? Good luck.

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call the molar mass of atom W as MM(W) (--read: molar mass of W)

A_xB_yC_z = x* MM(A) + y*MM(B) + z*MM(C)

example: C₆H₈N₂

The molar mass you have to get from periodic table, then eventually your memory--

but can't divine it, have to look it up first---see the table and find "molar mass" or

(synonymous) "atomic weight" of C , H , and N --you'll find around 12, 1, and 14 grams respectively.

so there you have 6 * MM(C) + 8 * MM(H) + 2 * MM(N) = 6*12 + 8*1 + 2*14 = 108

That 108 is in grams per mole of C₆H₈N₂ (see unit analysis below, if you want)

See it's like this--take Hydrgogen gas (H₂), for instance:

you have two atoms of Hydrogen in every molecule of H₂

you have two moles of atoms of H in ever mole of molecules of H₂

see? --> you have 2 moles of H atoms per mole of H₂

So if you want the grams per mole of H₂~ you need the grams per mole of H atoms, from the table,

and you multiply by 2 ... you get around 2 grams per mole of H₂

Okay--so how many grams is 0.5 moles ?--well it's have of the 2 grams that's in a mole: .5*2= 1 g .

[with units]how about 2 moles? --that's 2 moles * 2 grams per mole which is 4 grams (moles cancel)

how about 0.618 moles... well you have 0.618 moles * 2 grams / mole = 1.236 grams (ignoring sig figs)

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Here's the process worked out for Sodium Carbonate (Na₂CO₃)

You have 2 atoms of sodium, one of carbon, 3 of oxygen... per "molecule" of this salt

(ignore fact that they are sodium ions here-- matters not for our purposes;

electrons practically 0 mass, and anyway for every one missing there is one additional and net charge=0)

From the table, the molar masses are:

Na, around 23grams (you might see 22.99)

Carbon, around 12 g (you might see 12.011)

Oxygen, around 16 g (you might see 15.9999)

So in a mole of Sodium Carbonate you got 2 moles of Na at 23 grams per mole (2*23), and so on...

(see asterisk-bound section for generalized equation atop)... here's the whole thing with the so on included:

2 * 23 + 1*12 + 3*16 = 106

The units here in each term on the left are analogous to the case of sodium shown below:

2 moles Na atoms * 23 grams

----------------------- ----------------- + ....

mole Na₂CO₃ mole Na atoms

You can see the units "mole Na atoms" cancel and leaves it in units of grams per mole Na₂CO₃.

This will be true for moles carbon per mole our salt times grams per mole of carbon... and for oxygen too.

So the final answer of around 106 is in units of grams per mole of our salt.

We have 0.854 mole Na₂CO₃ * 106 grams / (Mole Na₂CO₃) = 90.5 (using three s.figs)