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Carbohydrate are compounds of carbon, hydrogen and oxygen in which the ratio of...

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bwang96 | Student, Grade 11 | (Level 1) Salutatorian

Posted April 21, 2013 at 3:55 AM via web

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Carbohydrate are compounds of carbon, hydrogen and oxygen in which the ratio of hydrogen atoms to oxygen atoms is 2:1. A certain carbohydrate is known to be 40% carbon. Determine the emperical formula for this carbohydrate.

1. Does the ratio means that the mass of hydrogen is twice as big as the oxygen?

2. I searched online for this question and the answer considers the hydrogen and oxygen as one thing,which is H2O for the mass, and use it to calculate the "n". But can we seperate hydrogen and oxygen when we calculate the mass?

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llltkl | College Teacher | (Level 3) Valedictorian

Posted April 21, 2013 at 6:18 AM (Answer #1)

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1. The ratio means that the number of atoms of hydrogen to oxygen is 2:1, the mass ratio of these two atoms may be obtained by multiplying atom ratio with their respectivecorresponding atomic masses. Here it will be 2×1.008 : 1×15.9994 = 1:8 (approximately).

2. Carbohydrates are known as ‘hydrates’ of carbon. This implies that, barring a few exceptions, they have the general formula Cx(H2O)y. So, ratio of the number of atoms of hydrogen to oxygen is always 2:1. That is the reason; hydrogen and oxygen are bracketed as one during calculations. However, this ratio can be worked out separately and independently if sufficient data are available. Sufficient means we need three separate equations to work out three separate (and independent) variables then.  

Returning to the original numerical problem, here the data is sufficient for framing two equations only. So, one has to take hydrogen and oxygen as dependent and hence one (in the form of H2O). The carbohydrate contains 40% carbon. So rest of it is the ‘hydrate’ part, i.e. H2O. Assuming these to be the mass ratio, atom/unit ratio is 40/12:60/18 = 3.3333:3.3333 = 1:1. So, the empirical formula of the carbohydrate is C1(H2O)1 or, CH2O.

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