Pure Na_2B_4O_7 is to be recovered from a solution containing 25 % Na_2B_4O_7. T

Question

Pure Na_2B_4O_7 is to be recovered from a solution containing 25 % Na_2B_4O_7. The solution is heated to 100 degreeC where 75 % of the water is removed and the resulting solution is cooled gradually to 55 degreeC where Na_2B_4O_7. 10H_2O crystals are produced. The crystals are separated by filtration and part of the liquor adheres to the filter cake. 5 g of the liquor adheres per 100 g crystals. The collected filter cake is sent to a dryer where all of the water were removed (including water of crystallization). Calculate the weight of the dry recovered. Solubility of at 55 degreeC is 14 g per 100 g water.

Explanation / Answer

MW of Na2B4O7 is 201.22 g/mol

When the solution contains 25 % Na2B4O7 in 100g 25 g is Na2B4O7 and 75 g is water

Now when it is heated to 100 oC 75% of the water is lost so water remaining is

0.75 x 75 = 56.25 g is lost and 18.75 g is left

Solubility of Na2B4O7 is 14 g/100g of water so Na2B4O7 soluble in 18.75 g of water is 2.625 g

So the total quantity is 100 - 56.25 = 43.75

SO the amount of Na2B4O7 that crystallizes has 25-2.625 = 22.375 g of Na2B4O7

So the amount of dry Na2B4O7 that is collected is 22.375 g per 100 g of the initial solution

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