9th Class Science: Is Matter Around Us Pure

Question: Calculation of a solution which contains 2.5 g. of salt dissolved in 50 g. of water.

Answer: Amount of salt = 2.5 g

Amount of water       = 50 g

Amount of solution   = 2.5 +50

= 52.5 g

Concentration of solution = 2.5/52.5×100

= 4.7%

Question: What is the concentration of solution which contains 16 g. Urea in 120 g. of solution.

Answer: Amount of Urea = 16 g

Amount of solution = 120 g

Concentration  = 16/120×100

= 13.3%

Question: Calculation concentration 5.6 ml alcohol+75 ml water.

Answer: Amount of alcohol = 5.6 ml

Amount of water = 75 ml

Amount of solution = 5.6+75

= 80.6 ml

Concentration  = 5.6/80.6×100

= 6.9%

Question: 25 ml of acetone is present in 150 ml. Of its aqueous solution, calculate concentration.

Answer: Amount of acetone = 25 ml

Amount of solution = 150 ml

Concentration  = 525/150×100

= 50/3

= 16.6%

Question: How much water should be added to 15 grams of salt to obtain 15 percent salt solution.

Answer: Let the total quantity be x

15% of x = 15 g

x = 15×100/15

= 100 g.

So, total [water+salt] = 100 g.

water = 100-15

= 85 grams

Question: How much water shout be mixed with 12 ml of alcohol to obtain 12% alcohol solution?

Answer: Let the total quantity be x

12% of x = 12 g.

x = 12×100/12

= 100

So, total (alcohol+water) = 100 g.

Water = total-alcohol

= (100-12) grams

= 88 grans