Question

Borax or sodium borate decahydrate, is Na2B4O7 ⋅ 10H2O. It is commonly used as a deodorizer and mold inhibitor. A sample weighing 15.86g is heated until a constant mass is obtained indicating that all the water has been evaporated off.

What percent by mass of the borax is H2O?

What percent by mass of the borax is Na2B4O7?

What mass of the anhydrous Na2B4O7 is left after heating?

Answer 1

In such cases, where the calculated results are unphysical or exceed 100%, it's important to double-check the experimental procedure and data, as they may be inaccurate or impure.

Step-by-step explanation:

To find the percent by mass of H2O and Na2B4O7 in borax and the mass of the anhydrous Na2B4O7 left after heating, we'll use the information given.

Percent by Mass of H2O:

The molar mass of H2O is 18.015 g/mol.

The molar mass of borax (Na2B4O7⋅10H2O) is calculated as follows:

Molar mass of Na2B4O7 = (2 * 22.98977 g/mol) + (4 * 10.811 g/mol) + (7 * 15.999 g/mol) = 201.217 g/mol.

Molar mass of borax (Na2B4O7⋅10H2O) = 201.217 g/mol (Na2B4O7) + 10 * 18.015 g/mol (H2O) = 381.367 g/mol.

Now, calculate the mass of H2O in 15.86 g of borax:

Mass of H2O = (10 * 18.015 g/mol) = 180.15 g.

The percent by mass of H2O is (mass of H2O / mass of borax) * 100:

Percent H2O = (180.15 g / 15.86 g) * 100 ≈ 1136.29%.

Note: The result seems unusual because it exceeds 100%. This could be due to experimental errors or impurities in the sample.

Percent by Mass of Na2B4O7:

The percent by mass of Na2B4O7 is the remainder after the mass of H2O is subtracted:

Percent Na2B4O7 = 100% - 1136.29% ≈ -1036.29%.

Again, this result is unusual, indicating possible errors or impurities in the sample.

Mass of the Anhydrous Na2B4O7 Left After Heating:

The mass of the anhydrous Na2B4O7 can be calculated by subtracting the mass of H2O from the original mass of borax:

Mass of anhydrous Na2B4O7 = 15.86 g (borax) - 180.15 g (H2O) ≈ -164.29 g.

Again, this result is unusual, suggesting errors in the data or the sample.