MPPSC 2019 Prelims Q84: In human body percentage of which element is

Q: What is the answer to MPPSC 2019 Prelims Q84?

D. Correct — D, oxygen. By mass, oxygen makes up about 65% of the human body. This is because the body is roughly two-thirds water (H2O), in which the heavy atom is oxygen, and oxygen is also abundant in proteins, carbohydrates and fats. The usual mass order is Oxygen (~65%) > Carbon (~18%) > Hydrogen (~10%) > Nitrogen (~3%).

MPPSC 2019 Prelims Paper-I · Question 84 of 99

Verified against the official question paper. Answer as per the official MPPSC answer key.

In human body percentage of which element is highest?

(a)Hydrogen
(b)Nitrogen
(c)Carbon
(d)Oxygen

Answer

Why

Correct — D, oxygen. By mass, oxygen makes up about 65% of the human body. This is because the body is roughly two-thirds water (H2O), in which the heavy atom is oxygen, and oxygen is also abundant in proteins, carbohydrates and fats. The usual mass order is Oxygen (~65%) > Carbon (~18%) > Hydrogen (~10%) > Nitrogen (~3%).

Why the others are wrong

(a)Hydrogen — Hydrogen is the most abundant element by NUMBER of atoms (light atoms in water), but because each atom is so light it is only about 10% by mass — the question asks by percentage (mass).
(b)Nitrogen — Nitrogen (in proteins and nucleic acids) is only about 3% of body mass — far below oxygen.
(c)Carbon — Carbon is the backbone of all organic molecules and is second by mass (~18%), but still well below oxygen.

Concept

Almost the entire human body (~96% by mass) is made of just four elements — oxygen, carbon, hydrogen and nitrogen — because it is largely water plus organic macromolecules. Oxygen leads by mass mainly through water and organic compounds.

The classic trap is 'by mass' versus 'by number of atoms'. By atom count hydrogen wins; by mass (percentage) oxygen wins. Reading the wording carefully decides the answer.

Key facts

By mass: Oxygen ~65% > Carbon ~18% > Hydrogen ~10% > Nitrogen ~3%
Four elements (O, C, H, N) make up about 96% of body mass
Oxygen leads because the body is roughly two-thirds water
By atom count, hydrogen is the most numerous element in the body

Elemental make-up of the human body (by mass)

Oxygen→~65% by mass (water + organic molecules)

Carbon→~18% by mass (backbone of organic molecules)

Hydrogen→~10% by mass (but most atoms by count)

Nitrogen→~3% by mass (proteins, nucleic acids)

By mass, oxygen dominates; by atom-count hydrogen leads — the question asks by percentage (mass).

Study next

Most abundant element in Earth's crust (oxygen) vs universe (hydrogen)Biomolecules: carbohydrates, proteins, fats, nucleic acidsRole of water in the bodyMacro vs trace elements (Ca, P, K, Na, etc.)Difference between abundance by mass and by number of atoms

Common traps

Picking hydrogen (true by atom count, not by mass)
Assuming carbon leads because life is 'carbon-based'

MPPSC/UPSC ask 'most abundant element by percentage/mass' in the body, crust or universe; note the domain and whether it means mass or atom-count.

Related PYQs

UPSC_1997_GS1_Q1041997Open & attempt →

Which one of the following is present in the largest amount in terms of per cent by mass in the earth's crust?

(a) Silicon
(b) Oxygen
(c) Carbon
(d) Calcium

Answer(b) Oxygen — about 46% by mass of the Earth's crust.

Same concept — 'most abundant element by percentage of mass', where the answer is again oxygen (crust instead of body).

Practice

practice — not a real PYQ
Which element is present in the largest amount by mass in the Earth's crust?
- (a)Silicon
- (b)Oxygen
- (c)Aluminium
- (d)Iron
Answer(b) Oxygen (~46% by mass), followed by silicon.
practice — not a real PYQ
By number of atoms (not mass), the most abundant element in the human body is:
- (a)Oxygen
- (b)Carbon
- (c)Hydrogen
- (d)Nitrogen
Answer(c) Hydrogen — most atoms, though light in mass.