By mass percentage

Chemistry Level 3

Each of the following combinations of elemental mass percentages are supposed to represent a single compound. Which combination is impossible?

A: 40% C, 6.67% H, 53.33% O

B: 21.70% Na, 11.32% C, 66.98% O

C: 76.73% C, 12.34% H, 10.93% O

D: 9.62% Na, 43.51% Cr, 46.87% O

E: 10.22% N, 2.92% H, 40.15% Mn, 46.71% O

A B C D E

Hobart Pao by Hobart Pao , 23, USA

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1 solution

Hobart Pao
Sep 19, 2016

The general procedure for getting the empirical formula given mass percentages is to pretend that you have 100 grams of the compound, and figure out how many moles of each element you have. Divide each number of moles obtained by the lowest number of moles in the combination of elements. Based on these formulas, you should be able to figure out which one is an impossible formula.

The whole solution will be updated when I get more time.

Choice A: 40 g C 12 g C ⋅ 1 mole C = 3.33 moles C \dfrac{40 g \, C }{12 g \, C} \cdot 1 \, \text{mole C} = 3.33 \, \text{moles C} 6.67 g H 1 g H ⋅ 1 mole H = 6.67 moles H \dfrac{6.67 g \, H}{1 g \, H} \cdot 1 \, \text{mole H} = 6.67 \, \text{moles H} 53.33 g O 16 g O ⋅ 1 mole O = 3.33 moles O \dfrac{53.33 g \, O}{16 g \, O} \cdot 1 \, \text{mole O} = 3.33 \, \text{moles O}

Dividing everything by 3.33, you get a 1:2:1 C:H:O ratio.

Empirical formula: C H 2 O CH_2 O This is possible--could be formaldehyde for example.

2 Helpful 0 Interesting 0 Brilliant 0 Confused

We are not to decide which compound is possible and which is not.. thus you have to include rhe first option too.

Archit Jain - 4 years, 8 months ago

my doubt was that you had solved the question according to the existence of the compound itself but it is not necessary for the compound to exist as the language certainly clear that the ratio should be whole no. therefore it might have more than one answer

Archit Jain - 4 years, 7 months ago

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each answer choice gives a different compound but one of the compounds isn't chemically possible.

Hobart Pao - 4 years, 7 months ago

sorry for the late reply

Archit Jain - 4 years, 7 months ago

no offence but just edit and add a that statement to your question "only chemically possible compounds" or rather add "only aldehydes" otherwise there would be confusion in the question..

Archit Jain - 4 years, 7 months ago

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