Chua Yong Qi's answer to mint's Junior College 1 H2 Maths Singapore question.
done
{{ upvoteCount }} Upvotes
clear
{{ downvoteCount * -1 }} Downvotes
since the question has stated that there are 2 carboxylic acid groups, you can calculate the Mr of the two groups first. since it is also stated that there are no C=C, the compound must contain n repeats of CH2 in the middle. using some algebra, you can solve for n and obtain the number of C atoms.
Date Posted:
3 years ago
The compound may not necesarily be a straight chain dicarboxylic acid with multiple methylene (-CH2-) groups linearly connected, since it was not specified. Branched acids are possible.
Eg. We could have HOOCC(CH2CH3)CH3COOH
or
HOOCC(H)(CH(CH3)2)COOH
What is certain is that the ratio of C to H for the rest of the molecule is always 1 : 2 .
Based on the general formula for a aliphatic monocarboxylic acid with all carbon-carbon bonds being single bonds :
CnH(2n+1)COOH
adding in a -COOH group to replace a H gives us : CnH2n(COOH)2
This can be treated as (CH2)n(COOH)2 when calculating the Mr.
Mr = n Mr(-CH2-) + 2 Mr (-COOH)
146 = n(12 + 2(1)) + 2(12 + 2(16) + 1)
146 = 14n + 90
14n = 56
n = 4
Number of carbons = 6
Fun fact : the acid in the question is adipic acid, with formula HOOC(CH2)4COOH
Eg. We could have HOOCC(CH2CH3)CH3COOH
or
HOOCC(H)(CH(CH3)2)COOH
What is certain is that the ratio of C to H for the rest of the molecule is always 1 : 2 .
Based on the general formula for a aliphatic monocarboxylic acid with all carbon-carbon bonds being single bonds :
CnH(2n+1)COOH
adding in a -COOH group to replace a H gives us : CnH2n(COOH)2
This can be treated as (CH2)n(COOH)2 when calculating the Mr.
Mr = n Mr(-CH2-) + 2 Mr (-COOH)
146 = n(12 + 2(1)) + 2(12 + 2(16) + 1)
146 = 14n + 90
14n = 56
n = 4
Number of carbons = 6
Fun fact : the acid in the question is adipic acid, with formula HOOC(CH2)4COOH
360 Tutoring Program