Question

part c. match the compound to the molar mass
draw lines to match each compound to its correct molar mass
compound \t molar mass (g/mol) \t correct √ or incorrect x \t correct molar mass (g/mol)
na₂so₄ \t 110.04 g/mol \t \t
cabr₂ \t 179.80 g/mol \t \t
al₂(co₃)₃ \t 107.92 g/mol \t \t
kbr \t 119 g/mol \t \t
one student calculated the molar masses below. check the student’s work and correct any errors.

Explanation:

Step1: Calculate molar mass of \( \text{Na}_2\text{SO}_4 \)

Molar mass = \( 2\times M(\text{Na}) + M(\text{S}) + 4\times M(\text{O}) \)
\( M(\text{Na}) = 22.99 \, \text{g/mol}, M(\text{S}) = 32.07 \, \text{g/mol}, M(\text{O}) = 16.00 \, \text{g/mol} \)
\( = 2\times22.99 + 32.07 + 4\times16.00 = 45.98 + 32.07 + 64.00 = 142.05 \, \text{g/mol} \) (Student's value 110.04 is incorrect)

Step2: Calculate molar mass of \( \text{CaBr}_2 \)

Molar mass = \( M(\text{Ca}) + 2\times M(\text{Br}) \)
\( M(\text{Ca}) = 40.08 \, \text{g/mol}, M(\text{Br}) = 79.90 \, \text{g/mol} \)
\( = 40.08 + 2\times79.90 = 40.08 + 159.80 = 199.88 \, \text{g/mol} \) (Student's value 179.80 is incorrect)

Step3: Calculate molar mass of \( \text{Al}_2(\text{CO}_3)_3 \)

Molar mass = \( 2\times M(\text{Al}) + 3\times(M(\text{C}) + 3\times M(\text{O})) \)
\( M(\text{Al}) = 26.98 \, \text{g/mol}, M(\text{C}) = 12.01 \, \text{g/mol}, M(\text{O}) = 16.00 \, \text{g/mol} \)
\( = 2\times26.98 + 3\times(12.01 + 48.00) = 53.96 + 3\times60.01 = 53.96 + 180.03 = 233.99 \, \text{g/mol} \) (Student's value 107.92 is incorrect)

Step4: Calculate molar mass of \( \text{KBr} \)

Molar mass = \( M(\text{K}) + M(\text{Br}) \)
\( M(\text{K}) = 39.10 \, \text{g/mol}, M(\text{Br}) = 79.90 \, \text{g/mol} \)
\( = 39.10 + 79.90 = 119.00 \, \text{g/mol} \) (Student's value 119 is correct, slight rounding)

Correct Molar Masses:

• \( \text{Na}_2\text{SO}_4 \): \( 142.05 \, \text{g/mol} \)
• \( \text{CaBr}_2 \): \( 199.88 \, \text{g/mol} \)
• \( \text{Al}_2(\text{CO}_3)_3 \): \( 233.99 \, \text{g/mol} \)
• \( \text{KBr} \): \( 119.00 \, \text{g/mol} \) (matches student's 119, so correct here)

For matching:

• \( \text{KBr} \) → 119 g/mol (correct)
• \( \text{Na}_2\text{SO}_4 \) → incorrect (110.04), correct ~142.05
• \( \text{CaBr}_2 \) → incorrect (179.80), correct ~199.88
• \( \text{Al}_2(\text{CO}_3)_3 \) → incorrect (107.92), correct ~233.99

Answer:

• \( \text{KBr} \) matches 119 g/mol (correct, mark √).
• \( \text{Na}_2\text{SO}_4 \): Student’s 110.04 is incorrect (mark X), correct molar mass ≈ 142.05 g/mol.
• \( \text{CaBr}_2 \): Student’s 179.80 is incorrect (mark X), correct molar mass ≈ 199.88 g/mol.
• \( \text{Al}_2(\text{CO}_3)_3 \): Student’s 107.92 is incorrect (mark X), correct molar mass ≈ 233.99 g/mol.

(For matching, draw lines: \( \text{KBr} \) to 119 g/mol; others have incorrect student values, so their correct molar masses are as calculated above.)