Characteristics of Particles of MatterPractice Questions & MCQs

Q1. A student dissolves 2 g of potassium permanganate crystals in 100 mL of water and then draws off 10 mL, topping it back up to 100 mL with fresh water for the very first round. After this one dilution, how many grams of the salt are present in each mL of the new solution?

• A.0.002 g/mL
• B.0.02 g/mL
• C.0.2 g/mL
• D.0.0002 g/mL

Answer: A. 0.002 g/mL

Start: 2 g in 100 mL. Taking 10 mL and refilling to 100 mL dilutes by 100/10 = 10 times. New concentration = (2/100)/10 = 0.002 g/mL.

Q2. In a school lab a pinch of copper sulphate weighing 5 g is stirred into 100 mL of water, after which 10 mL is withdrawn and the beaker is filled back to 100 mL just once. After this one dilution, how many grams of the salt are present in each mL of the new solution?

• A.0.005 g/mL
• B.0.05 g/mL
• C.0.5 g/mL
• D.0.0005 g/mL

Answer: A. 0.005 g/mL

Start: 5 g in 100 mL. Taking 10 mL and refilling to 100 mL dilutes by 100/10 = 10 times. New concentration = (5/100)/10 = 0.005 g/mL.

Q3. A blue copper sulphate mixture begun from 5 g of salt in 100 mL is thinned again and again by scooping out 10 mL and restoring the level to 100 mL each time; even after 4 such rounds the colour can still be made out. Overall, how many times more dilute is the final batch than the starting one?

• A.10 times
• B.1000 times
• C.10000 times
• D.40 times

Answer: C. 10000 times

Each round dilutes by 10. Over 4 rounds the factors multiply: 10^4 = 10000. That the tint persists shows one crystal holds an enormous number of tiny particles.