Which element has a mass percent composition of 21.49% in borax (Na2B4O7·10H2O)?

• A. Sodium
• B. Hydrogen
• C. Boron
• D. Oxygen

Answer: (C)

Verified step by step guidance

1. Identify the chemical formula of borax, which is given as \(\mathrm{Na_2B_4O_7 \cdot 10H_2O}\).
2. Calculate the molar mass of each element in the formula by multiplying the number of atoms of each element by its atomic mass: sodium (Na), boron (B), oxygen (O), and hydrogen (H). Remember to include the water of hydration (10 \(\mathrm{H_2O}\)) in your calculation.
3. Sum the molar masses of all atoms in the formula to find the total molar mass of borax.
4. Calculate the total mass contributed by boron atoms by multiplying the number of boron atoms (4) by the atomic mass of boron.
5. Determine the mass percent of boron in borax using the formula: \(\text{Mass \% of B} = \left( \frac{\text{Mass of B}}{\text{Total molar mass of borax}} \right) \times 100\%\).