Question

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A compound 'A' has the following percentage composition by mass:-

Carbon 26.7% , Oxygen 71.1% , Hydrogen 2.2%. Determine the empirical formula of A. If the relative molecular mass of A is 90, what is the molecular formula of A?

[ H = 1, C = 12, O =16]

Answer 1

heya⤵️⤵️⤵️
First you have to assume that the total amount of grams that you have is 100.

This would mean that Carbon has 26.7 grams, Hydrogen has 2.2 grams, and Oxygen has 71.1 grams.

Now, convert these numbers to moles:

Carbon = 2.22 moles

Hydrogen = 2.18 moles

Oxygen = 4.4 moles

Now, divide by the smallest, which is Hydrogen (round to the nearest whole number):

2.22/ 2.18 = 1

2.18/2.18 = 1

4.4/ 2.18 = 2

These are the moles within the empirical formula of the compound (the simplest formula):

HCO2

But the question isn't asking for the empirical formula, it is asking for the true formula that gives you a molecular weight of 90- so you simply multiply by whole numbers until you get your answer.

If this were a more complicated problem, you could try a lot of different formulas, such as:

H2C2O4 (multiplied by 2)

H3C3O6 (multiplied by 3)

And so on and so forth. But for this problem, multiplying by two will suffice.

H x 2 = 1.008 x 2 = 2.016

C x 2 = 12.01 x 2 = 24.02

O2 x 2 = 32 x 2 = 64

64 + 24.02 + 2.016 = 90.036

So your answer is H2C2O4

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Answer 2

First you have to assume that the total amount of grams that you have is 100.

This would mean that Carbon has 26.7 grams, Hydrogen has 2.2 grams, and Oxygen has 71.1 grams.

Now, convert these numbers to moles:

Carbon = 2.22 moles

Hydrogen = 2.18 moles

Oxygen = 4.4 moles

Now, divide by the smallest, which is Hydrogen (round to the nearest whole number):

2.22/ 2.18 = 1

2.18/2.18 = 1

4.4/ 2.18 = 2

These are the moles within the empirical formula of the compound (the simplest formula):

HCO2

But the question isn't asking for the empirical formula, it is asking for the true formula that gives you a molecular weight of 90- so you simply multiply by whole numbers until you get your answer.

If this were a more complicated problem, you could try a lot of different formulas, such as:

H2C2O4 (multiplied by 2)

H3C3O6 (multiplied by 3)

And so on and so forth. But for this problem, multiplying by two will suffice.

H x 2 = 1.008 x 2 = 2.016

C x 2 = 12.01 x 2 = 24.02

O2 x 2 = 32 x 2 = 64

64 + 24.02 + 2.016 = 90.036

So your answer is H2C2O4