Chemistry · Solutions · NEET
It is the whole solution, not just the solvent. Total mass of solution = mass of solute + mass of solvent. So mass % of solute = (mass of solute) / (mass of solute + mass of solvent) x 100. Many students wrongly divide by only the solvent mass. Example: 10 g glucose in 90 g water gives 10/(10+90) x 100 = 10%, not 10/90.
Mass % uses masses (in grams). Volume % uses volumes (in mL). Mass % = (mass of component / total mass of solution) x 100. Volume % = (volume of component / total volume of solution) x 100. Use mass % for solids in liquids (like sugar in water). Use volume % for liquid-in-liquid mixtures (like ethanol in water). NEET often gives you density to switch between the two.
ppm means parts per million. You use it when the solute is present in a very tiny (trace) amount, so a normal percentage would be a very small number. ppm = (parts of the component / total parts of all components) x 10^6. It can be by mass or by moles. Example from NCERT: 1 ppm of fluoride ions in water helps prevent tooth decay. Rule of thumb: 1 ppm is about 1 mg of solute in 1 kg (1 litre) of a dilute water solution.
Mass by volume percentage is the mass of solute (in grams) present in 100 mL of solution. Formula: w/V % = (mass of solute in g / volume of solution in mL) x 100. It is used a lot in medicine and pharmacy. So a 5% (w/V) glucose drip means 5 g of glucose in every 100 mL of solution.
For dilute water solutions the density is close to 1 g/mL, so 1 L of solution weighs about 1000 g = 1,000,000 mg. Then 1 ppm by mass = 1 mg of solute in 1,000,000 mg of solution = 1 mg per litre. So for water, ppm and mg/L are almost the same number. This shortcut saves time in NEET when the question mixes ppm and mg/L.
No. Mass %, ppm, molality and mole fraction all use mass or number of particles, which do NOT change when temperature changes. Volume % and molarity DO change with temperature because volume expands or shrinks when heated. This is a very common NEET trap, so remember: anything with volume is temperature-dependent.
Identify the correct statements: (A) The molality of 2.5 g of ethanoic acid (Molar mass 60 g/mol) in 75 g of benzene is 0.556 m. (B) The molarity of a solution containing 5 g of NaOH (molar mass 40 g/mol) in 450 mL of solution is 0.278 M. (C) Aquatic species are more comfortable in cold water. (D) The solubility of a gas increases with decrease in pressure. (E) For a binary mixture of A and B with moles n_A and n_B, the mole fraction of B is x_B = n_A/(n_A + n_B).
The density of a 2 M aqueous solution of NaOH is 1.28 g/cm^3. The molality of the solution is (molar mass of NaOH = 40 g/mol):
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Mass % of a component = (mass of that component in the solution / total mass of the solution) x 100. The total mass is the mass of solute plus the mass of solvent.
ppm = (number of parts of the component / total number of parts of all components) x 10^6. It can be measured by mass or by number of moles, and is used for very dilute (trace) solutions.
For dilute water solutions, yes, approximately. Because water has a density near 1 g/mL, 1 litre weighs about 1,000,000 mg, so 1 ppm by mass equals about 1 mg per litre.
Mass %, ppm, molality and mole fraction do not change with temperature because they use mass or number of particles. Volume % and molarity do change, because volume changes with temperature.
w/V (mass by volume) percentage is the mass of solute in grams present in 100 mL of solution. A 5% (w/V) solution has 5 g of solute in every 100 mL of solution.
