The compound C₆H₁₂O₄ contains
- (a)22 atoms per mole
- (b)twice the mass percent of H as compared to the mass percent of C
- (c)six times the mass percent of C as compared to the mass percent of H
- (d)thrice the mass percent of H as compared to the mass percent of O
Correct — C, six times the mass percent of C as compared to the mass percent of H. Take the formula exactly as the booklet prints it. Its molar mass is 6 times 12 for carbon, 12 times 1 for hydrogen and 4 times 16 for oxygen, that is 72 plus 12 plus 64, or 148 grams per mole. Carbon therefore makes up 72 out of 148, about 48·6 per cent, hydrogen 12 out of 148, about 8·1 per cent, and oxygen 64 out of 148, about 43·2 per cent. The carbon share divided by the hydrogen share is 72 divided by 12, exactly 6. There is a shortcut worth keeping: since the percentages share the same denominator, the ratio of mass percentages is simply the ratio of the masses contributed, 72 to 12, and the 148 never has to be worked out at all.
- (a)22 atoms per mole — The arithmetic behind the number is right — 6 carbon plus 12 hydrogen plus 4 oxygen makes 22 atoms — but those 22 atoms are what one molecule contains, not what one mole contains. A mole of the compound holds 22 moles of atoms, which is 22 multiplied by about 6·022 times ten to the twenty-three atoms. Read strictly, the statement is off by Avogadro's number, and that is why the key marks the mass-percentage statement instead.
- (b)twice the mass percent of H as compared to the mass percent of C — This inverts the true relation and changes its size. Hydrogen contributes about 8·1 per cent and carbon about 48·6 per cent, so hydrogen is one-sixth of carbon, not twice it. Twelve hydrogen atoms weigh only 12 units against carbon's 72 because each hydrogen atom is twelve times lighter than each carbon atom — more atoms need not mean more mass.
- (d)thrice the mass percent of H as compared to the mass percent of O — Hydrogen contributes 12 mass units and oxygen 64, so oxygen's share of about 43·2 per cent is roughly 5·3 times hydrogen's 8·1 per cent. Hydrogen is a small fraction of oxygen here, nowhere near three times it.
The mass percentage of an element in a compound is the mass that element contributes to one mole, divided by the molar mass of the compound, times a hundred. Because every element in a given compound is divided by the same molar mass, comparing two elements needs only their contributed masses — the number of atoms multiplied by the atomic mass. A molecular formula also fixes the atom count per molecule, and multiplying by Avogadro's number turns that into the atom count per mole.
One printed detail should be stated plainly rather than quietly corrected. The booklet prints the formula as C₆H₁₂O₄, which is not the formula of glucose; glucose is C₆H₁₂O₆. The printed formula has been checked against the paper at high resolution and it is what the question sets, and the paper's own option (a) confirms it, since 6 plus 12 plus 4 comes to the 22 that option quotes. Work the item as printed. It is worth noting for reassurance that the carbon-to-hydrogen mass ratio would be 72 to 12, that is 6, for C₆H₁₂O₆ as well, so the marked answer survives either reading of the formula — it is only option (a) that depends on the fourth oxygen.
- The molar mass of C₆H₁₂O₄ is 148 grams per mole, from 72 plus 12 plus 64.
- Its mass percentages are about 48·6 per cent carbon, 8·1 per cent hydrogen and 43·2 per cent oxygen.
- The ratio of two elements' mass percentages equals the ratio of their contributed masses, so the molar mass cancels out.
- One mole of the compound contains 22 moles of atoms, that is about 22 times 6·022 times ten to the twenty-three atoms.
- The formula printed in this paper is C₆H₁₂O₄, which is not glucose; glucose is C₆H₁₂O₆.
Carbon's mass percentage is exactly six times hydrogen's — option (c).
- Reading a per-molecule atom count as a per-mole atom count; the two differ by Avogadro's number.
- Assuming the element with the most atoms contributes the most mass — hydrogen has twelve atoms here and the smallest mass share.
- Working out the molar mass when only a ratio of percentages is asked for; the denominator cancels.
NDA sets percentage-composition items either as this kind of comparative statement or as a direct request for the percentage of one element in a named compound.
Which one of the following is the correct sequence in increasing order of molecular weights of the hydrocarbons?
- (a) Methane, ethane, propane and butane
- (b) Propane, butane, ethane and methane
- (c) Butane, ethane, propane and methane
- (d) Butane, propane, ethane and methane
Answer(a) Methane, ethane, propane and butane
Runs the same first step — reading a molecular formula and turning it into a molar mass by multiplying each subscript by its atomic mass.
How many moles of hydrogen atom are present in one mole of Aluminium hydroxide ?
- (a) One mole
- (b) Two moles
- (c) Three moles
- (d) Four moles
Answer(c) Three moles
Puts the mole-against-molecule distinction directly on the table — the subscript counts atoms in a molecule and moles of atoms in a mole, which is exactly the reading that makes option (a) of the 2017 item fail.
20 g of common salt is dissolved in 180 g of water. What is the mass percentage of the salt in the solution ?
- (a) 5%
- (b) 9%
- (c) 10%
- (d) 15%
Answer(c) 10%
The same mass-percentage arithmetic applied to a solution rather than a compound — part divided by whole, times a hundred.
- practice — not a real PYQ
The mass percentage of oxygen in C₆H₁₂O₄, whose molar mass is 148 g mol⁻¹, is closest to
- (a)8 per cent
- (b)27 per cent
- (c)43 per cent
- (d)49 per cent
Answer(c) 43 per cent — oxygen contributes 4 × 16 = 64 units out of 148, which is about 43·2 per cent.
- practice — not a real PYQ
How many moles of hydrogen atoms are present in one mole of C₆H₁₂O₄?
- (a)1 mole
- (b)6 moles
- (c)12 moles
- (d)22 moles
Answer(c) 12 moles — the subscript gives twelve hydrogen atoms per molecule, so twelve moles of hydrogen atoms per mole of compound.