Question

The volume of 0.25 M NaOH to be added to 250 mL of 0.15 M NaOH so that the resultant
solution is 0.2 M would be
1. 250 mL 2. 350 mL 3. 450 mL 4. 550 mL​

Answer 1

Answer: option 1 (250 mL)

Explanation:

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Answer 2

The volume of 0.25M NaOH would be 250ml. hence the correct option is (1) 250 ml.

We have to find the volume of 0.25M NaOH to be added to 250ml of 0.15M NaOH so that resultant solution is 0.2M.

1. 250 ml
2. 350 ml
3. 450 ml
4. 550 ml

CONCEPT OF DILUTION : if we dilute a solution, its volume changes and even its concentration changes but the number of moles of solute remains constant.

i.e., initial no of moles of solute = final no of moles of solute

We also know,

no of moles of solute = molarity of solute × volume of solution.

Let volume of 0.25M NaOH is x ml.

so number of moles of solute in 0.25 M NaOH = 0.25M × x mL

and number of moles of solute in 0.15M NaOH = 0.15M × 250 mL

and number of moles of solute in resultant solution = 0.2M × (x + 250) mL

From the concept of dilution,

Initial no of moles of solute = final no of moles of solute

⇒ 0.25M × x mL + 0.15M × 250 mL = 0.2M × (x + 250)mL

⇒ 0.25x + 37.5 = 0.2x + 50

⇒ 0.05x = 50 - 37.5 = 12.5

⇒ x = 250 ml

Therefore the volume of 0.25M NaOH is 250ml.

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