Weigh out accurately to the nearest 0.1 mg about 0.1 g of the basic oxide provid

Question

Weigh out accurately to the nearest 0.1 mg about 0.1 g of the basic oxide provided and transfer the sample carefully to a 125 ml. Erlenmeyer flask. Now carefully add with a pipet or buret exactly 25.00 ml. of the standard HC1 solution. Swirl the flask gently until all of the oxide has reacted. Once all the oxide has reacted, the solution should be clear. However, it may be necessary to warm the mixture to ensure that the sample has reacted completely. Cool to room temperature if necessary, add 4 drops of phenolphthalein indicator, and titrate with the standard NaOH solution until a faint pink color is obtained. Repeat the determination with another accurately sample of the basic oxide, from your results calculate the equivalent weight of a basic oxide.

Explanation / Answer

First let's assume that the basic oxide is represented as BO and its reaction with HCl is as follows :

BO + 2HCl ---> BCl2 + H2O

If the molarity of standard HCl solution is 'm', and assume that 'x' ml of NaOH is used in the titration to get the pink color.

Since NaOH and HCl react in 1:1 ratio, this means that (25-x)ml HCl was consumed in the reaction with basic oxide.

From the molarity we calculate the moles of HCl reacted, which is : (25-x)/1000 * m .

From the balanced reaction, we see that 2 moles of HCl react with 1 mole of basic oxide, this means that moles of oxide present are [ (25-x)/1000 * m ] / 2 .

Molar mass is calculated by dividing 0.1 with the above calculated figure.

From this molar mass we can calculate equivalent mass. In our case since we have assumed the basic oxide as BO, this means that molar mass and equivalent mass are the same for BO.

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