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12.0g of a mixture of potassium carbonate and potassium chloride were

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CHEMISTRY

12.0g of a mixture of potassium carbonate and potassium chloride were dissolved in a 250cm³ standard flask. 25cm³ of this solution required 40.00cm³ of 0.1 M HCI neutralization. What is the percentage by weight of K₂CO₃ in the mixture?

(K = 39, O = 16, C = 12)

A. 60

B. 72

C. 82

D. 92

QUICK ANSWER…

D

DETAILS…  

Please note that:

Potassium carbonate is a base which will react and be neutralized by the acid HCl,

Potassium chloride is a salt which will remain inactive while the reaction occurs.

Equation of neutralization:

K₂CO₃ + 2HCl → 2KCl + H₂O + CO₂

We are going to be using acid – base equation to determine the molar concentration of the base.

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12.0g of a mixture of potassium carbonate and potassium chloride were

What this simply mean is that the molar concentration of the base K₂CO₃ in the solution is 0.08 mol/cm³

But, mass concentration = molar concentration x molar mass

Molar mass of K₂CO₃ = 2(39) + 12 + 3(16) = 138g/mol

Therefore, mass concentration = 0.08 x 138 = 11.04g/cm³

But the mass of the mixture = 12 g/cm³

Hence, the percentage by mass/weight of K₂CO in the mixture = (11.04/12) × 100 = 92%

Now for the right answer to the above question:

  1. Option A is incorrect.
  2. Option B is incorrect.
  3. C is incorrect.
  4. D is the correct answer.

KEY-POINTS…

You may please note these/this:

  • Remember, the 250cm3 is not used in our calculation because it is merely the size of the flask, and we were not told it was filled to the brim.

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/ culled from 2015 JAMB-UTME Chemistry past question 8 /

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