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3. In the gravimetric determination of sulfate in a 0.2841 g sample of pure Na₂SO, a BaSO4, precipitate weighing 0.4604 g was obtained. The weight of the precipitate was smaller than the theoretical one since some BaSO4 was converted to Bas during the heating process.

(a) Calculate the percent of BaS in the precipitate (Hint: best to solve algebraically)
(b) Calculate the percent error of the analysis (Hint: compare calculated with weight stated in
problem).

User Hsivonen
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1 Answer

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To solve this problem, we need to follow these steps:

(a) Calculate the percent of BaS in the precipitate:

Let's denote:

- W₁ = weight of BaSO₄ precipitate obtained (0.4604 g)

- W₂ = weight of BaS formed during the heating process

We know that the total initial weight of the compound was 0.2841 g (Na₂SO₄). After the heating process, some BaSO₄ converted to BaS, so the total weight will still be 0.2841 g:

0.4604 g (BaSO₄) + W₂ (BaS) = 0.2841 g (initial weight)

Solving for W₂:

W₂ = 0.2841 g - 0.4604 g

W₂ = -0.1763 g

This negative weight doesn't make physical sense in this context, indicating there might be an error in the problem setup or data.

(b) Calculate the percent error of the analysis:

Percent error is calculated using the formula:

Percent Error = (|Experimental Value - Theoretical Value| / Theoretical Value) * 100

In this case, the experimental value is the weight of the precipitate obtained (0.4604 g), and the theoretical value would be the expected weight of BaSO₄ if all the BaSO₄ in the sample had precipitated and none had converted to BaS.

The theoretical weight of BaSO₄ can be calculated by considering the molar mass of Na₂SO₄ and BaSO₄:

Molar mass of Na₂SO₄ = 22.99 * 2 + 32.07 + 15.999 * 4 = 142.04 g/mol

Molar mass of BaSO₄ = 137.33 + 32.07 + 15.999 * 4 = 233.39 g/mol

Using the molar ratios of Na₂SO₄ and BaSO₄, the theoretical weight of BaSO₄ in the sample would be:

Theoretical Weight of BaSO₄ = (Molar mass of BaSO₄ / Molar mass of Na₂SO₄) * Initial Weight of the Sample

Theoretical Weight of BaSO₄ = (233.39 g/mol / 142.04 g/mol) * 0.2841 g

Theoretical Weight of BaSO₄ = 0.4665 g

Now we can calculate the percent error:

Percent Error = (|0.4604 g - 0.4665 g| / 0.4665 g) * 100

Solving this will give you the percent error of the analysis.

Please note that the negative value obtained for the weight of BaS in part (a) indicates a problem with the problem setup or data provided. Double-check the values and equations to ensure accuracy.

User Flores
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