JEE Main 2021 · 24 February · morning · Numerical answer · +4 / −0
4.5 g of compound A (MW = 90) was used to make 250 mL of its aqueous solution. The molarity of the solution in M is x × 10⁻¹.
The value of x is _______. (Rounded off to the nearest integer)
Answer and solution
Answer 2
Solution
Chemistry ma’am · 3 steps
Turn grams into moles first, then divide by the volume in litres, not millilitres.
- Moles of A. Divide the mass by the molar mass: n = 4.5/90 = 0.05 mol.
- Volume in litres. Molarity is moles per litre, so convert: 250 mL = 0.25 L.
M = n/V = (0.05 mol)/(0.25 L) = 0.2 M
- So. 0.2 M = 2 × 10⁻¹ M, so x = 2.
The common slip: 0. Dividing by 250 instead of 0.25 gives 2 × 10⁻⁴ M, and reading off 0.2 as x without matching the 10⁻¹ factor gives 0 after rounding.
The idea to keep. Molarity always needs moles on top and litres below. Convert mL to L before you divide, and match your answer to the power of ten the question asks for.
Answer from the NTA final answer key. Solution by PrepWiser.