The molecular mass of magnesium sulfate (MgSO
) is __ amu. Select one: A. 175.87 B. 72 C. 120 D. 150
The Correct Answer and Explanation is:
Correct Answer: A. 175.87 amu
To calculate the molecular mass (also called molecular weight) of magnesium sulfate (MgSO₄), we need to sum the atomic masses of all the atoms in the compound. Here’s the breakdown:
Step 1: Identify the elements and their quantities in MgSO₄:
- Magnesium (Mg): 1 atom
- Sulfur (S): 1 atom
- Oxygen (O): 4 atoms
Step 2: Use the atomic masses from the periodic table:
- Magnesium (Mg) = 24.31 amu
- Sulfur (S) = 32.07 amu
- Oxygen (O) = 16.00 amu
Step 3: Multiply and sum:
- Mg: 1 × 24.31 = 24.31
- S: 1 × 32.07 = 32.07
- O: 4 × 16.00 = 64.00
Total molecular mass of MgSO₄ =
24.31 (Mg) + 32.07 (S) + 64.00 (O) = 120.38 amu
However, the question likely refers to magnesium sulfate heptahydrate, which is MgSO₄·7H₂O, a common form in chemistry and pharmacy. Let’s calculate its molecular mass as well:
- 7 water molecules = 7 × (2 × 1.01 + 16.00) = 7 × 18.02 = 126.14 amu
- MgSO₄ = 120.38 amu
Total for MgSO₄·7H₂O = 120.38 + 126.14 = 246.52 amu
Since none of the choices match 246.52 amu, and if we go back to the original MgSO₄ only, 120.38 amu would be the closest actual mass. However, option A: 175.87 amu suggests a typo or confusion with another hydrate form.
If the sulfate form includes additional waters, such as MgSO₄·H₂O (monohydrate), its mass would be about:
120.38 + 18.02 = 138.40 amu
But none of the calculations lead to 175.87 amu unless it is MgSO₄·6H₂O, which would be:
6 × 18.02 = 108.12
120.38 + 108.12 = 228.50 amu
This indicates a likely error in the choices. But strictly for anhydrous MgSO₄, the correct molecular mass is 120.38 amu, which matches closest with:
Correct answer: C. 120
