Q4. A litre of sea water of mass 1030 g contains 6.0 × 10−3 g of dissolved oxygen. What is this in ppm by mass?
6.0 × 10−3/1030 × 106 = 5.8 ppm, as NCERT gives (page 3).
Q5. A commercial bleaching solution contains 3.62 mass percentage of sodium hypochlorite. How much sodium hypochlorite is in 500 g of it?
3.62/100 × 500 = 18.1 g (percentage from NCERT page 2).
Q6. Assertion: the molality of a solution does not change when it is warmed. Reason: molality is defined using masses, which do not change with temperature. Are both true, and does the reason explain the assertion?
Both are true for a closed sample of unchanged composition; evaporation, reaction or precipitation would require recalculating the composition (NCERT page 5).
Q7. A 0.50 M aqueous glucose solution (molar mass 180 g mol−1) has a density of 1.05 g mL−1. Find its molality.
One litre has a mass of 1050 g, of which 0.50 × 180 = 90 g is glucose, leaving 960 g of water. m = 0.50/0.960 = 0.521 mol kg−1 (beyond the NCERT text).
Q8. 46 g of ethanol (C2H5OH) is mixed with 36 g of water. Find the mole fraction of ethanol.
Ethanol: 46/46 = 1 mol; water: 36/18 = 2 mol. xethanol = 1/3 = 0.333 (NCERT Eq. (1.5), page 3).
Q9. Find the mole fraction of the solute in a 1.00 mol kg−1 aqueous solution.
1 mol of solute goes with 1000/18 = 55.56 mol of water. xsolute = 1/(1 + 55.56) = 0.0177 (NCERT Eqs. (1.5) and (1.9)).