Question

Saloni prepared 250 g of salt solution with concentration 25%(w/w). She added 25g more salt to this solution and evaporated 70g water . Now the final concentration of solution will be :.
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Answer 1

Answer:

The key is to recognize the percentage concentration is mass of solute divided by mass of solution, not the original solvent.

If you add x grams of salt, the solution has a mass of 250 g + x

x/(250 g + x) = 0.15

x = 250 g * 0.15/0.85 = 44.12 g (rounded)

Check 44.12/294.12 = 0.150007, close enough, given rounding.

Answer 2

Given: Saloni prepared 250 g of salt solution with concentration 25% (w/w). She added 25 g more salt to this solution and evaporated 70 g water.

To find: The final concentration of the solution.

Solution:

• Concentration of a salt solution is given by the following formula,

        $concentration = \frac{w_{1}}{w_{1} + w_{2}} * 100$

• Here, w₁ is the weight of the solute (salt) and w₂ is the weight of the solvent (water) in the solution. So, w₁ + w₂ is the weight of the solution.

• Let the initial weight of the salt be x and it can be calculated as,

        $25 = \frac{x}{250 g } * 100$

        $x = 62.5 g$

• The initial weight of the water is,

        $250 - 62.5 = 187.5 g$

• Since more 25 g of salt is added to the solution, the new weight of the salt is,

        $62.5 + 25 = 87.5 g$

• Since 70 g of the water is evaporated, its new weight is,

        $187.5 - 70 = 117.5 g$

• So, the new concentration can be calculated as,

        $concentration = \frac{87.5}{87.5 + 117.5} * 100$

                                $= 42.68 %$

Therefore, the final concentration of the solution will be 42.68 %.