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# Impossible empirical formula!!!

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1. I can't seem to work this out?
Can someone guide me step by step?

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2. (Original post by DatSmartGuy)
I can't seem to work this out?
Can someone guide me step by step?

Posted from TSR Mobile
Hey,

Firstly you work out the moles which is mass divided by Mr

So Fe: 6.21/207 = 0.03 moles
Oxygen: 0.72/16 = 0.045 moles

Therefore, you have a ratio of:
0.03 : 0.045

Then, you divide by the smallest number of moles - in this case you will divide both numbers by 0.03. Leaving you with:

1 : 1.5

You can't have 1.5 of an element so you X2 on the right to get a whole number of 3. But what you do on the right you do to the left. So you times 1 by 2 leaving you with:

2 : 3

Pb203 (the 2 & 3 are below the letters)

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3. First, take the the experimental masses you are given:

Oxygen - 0.72 grams, Lead - 6.21 grams

Then, divide these by the relative atomic mass:

0.72 divided by 16 = 0.045
6.21 divided by 207 = 0.03

Then, arrange them into a ratio:

0.045:0.03, which is 45:30, which is 3:2

So 3 Oxygen atoms and 2 Lead atoms.

Make sure to put them into the right order, the formula asks for PbO, so the answer is Pb203 (2 and 3 should be small numbers)
4. DatSmartGuy

1) For lead:
6.21/207 = 0.03

For oxygen:
0.72/16 = 0.045

2) Divide the values for these two by the smallest one:
Lead: 0.03/0.03 = 1
Oxygen: 0.045/0.03 = 1.5

3) Pb2O3 - get this when you get the ratios into whole numbers.
5. Moles of Pb = 6.21/207 = 0.03 moles
Moles of O = 0.72/16 = 0.045
Divide by smallest = Divide both by 0.03

0.03/0.03 = 1 Pb
0.045/0.03 = 1.5 O
However empirical formula is the smallest WHOLE NUMBER ratio of atoms of each element in a compound.
Therefore multiply both by 2

= 2 Pb and 3 O
= Pb2O3
6. (Original post by z1820)
Hey,

Firstly you work out the moles which is mass divided by Mr

So Fe: 6.21/207 = 0.03 moles
Oxygen: 0.72/16 = 0.045 moles

Therefore, you have a ratio of:
0.03 : 0.045

Then, you divide by the smallest number of moles - in this case you will divide both numbers by 0.03. Leaving you with:

1 : 1.5

You can't have 1.5 of an element so you X2 on the right to get a whole number of 3. But what you do on the right you do to the left. So you times 1 by 2 leaving you with:

2 : 3

Pb203 (the 2 & 3 are below the letters)

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Thank you very much

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7. Thank you to everyone that responded it really helped me out.

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