Question

How to calculate: Prepare 10mL of 5% (w/v) K2CrO4 indicator solution

Answer 1

First of all w/v means, Weight by volume
Which means that much grams or killograms of solute is present in 100 mililitre or litre of solution respectively.
For example: If we will say 10% (w/v)NaOH solution, then it means 10 grams of NaOH is present in 100 ml water(as not mentioned, the solvent is always water)
or we can say 10 Kg of NaOH is present in 100 litres water

here, we need to prepare 10ml of 5% w/v K2CrO4 solution:
so for 5% K2CrO4 solution , 5 grams of K2CrO4 should be present in 100 ml water, so for preparing a 10 ml solution, we need to use 0.5 grams of K2CrO4 in 10 ml Water.

Again. molar mass of K2CrO4 is 194.1896 grams/mol
which means 1 mole of K2CrO4 weighs 194.1896 grams
so, to get 0.5 grams K2CrO4, we need 0.5/194.1896 = 0.002574 moles
so 0.002574 moles of K2CrO4 should be used in 10 ml water to get 5% (w/v) K2CrO4 solution.