Be sure to answer all parts. Select the formula for magnesium nitrite trihydrate and determine its molecular (formula) mass.
Formula:
Mg(NO2)2·3H2O
Molecular Mass: amu
The Correct Answer and Explanation is:
Formula:
Mg(NO₂)₂·3H₂O
Molecular Mass:
148.53 amu
Explanation
Magnesium nitrite trihydrate is an ionic compound consisting of magnesium cations, nitrite anions, and water of hydration. Understanding its formula and calculating its molecular mass involves knowledge of chemical nomenclature and atomic masses.
Step 1: Understanding the Formula
- Magnesium (Mg) is a Group 2 element, so it forms a +2 ion: Mg²⁺.
- Nitrite (NO₂⁻) is a polyatomic ion with a −1 charge.
- To balance charges, two nitrite ions are needed for every magnesium ion:
Mg²⁺ + 2 NO₂⁻ → Mg(NO₂)₂ - The ·3H₂O indicates three water molecules of hydration, which are physically associated with the compound but not part of the ionic lattice. These are known as waters of crystallization or hydrates.
Thus, the full formula is Mg(NO₂)₂·3H₂O, representing magnesium nitrite trihydrate.
Step 2: Calculating Molecular Mass
To calculate the molecular (formula) mass, sum the atomic masses of all atoms in the compound:
- Mg = 24.31 amu
- N = 14.01 amu × 2 = 28.02 amu
- O in NO₂ = 16.00 amu × 4 = 64.00 amu
(There are 2 NO₂⁻ ions, each with 2 O atoms) - H₂O = (2 × 1.008) + 16.00 = 18.02 amu × 3 = 54.06 amu
Total Molecular Mass =
24.31 (Mg) + 28.02 (N) + 64.00 (O from NO₂) + 54.06 (H₂O)
= 170.39 amu
Correction: Above sum gives 170.39 amu, not 148.53 amu.
Let’s re-calculate carefully:
- Mg: 24.31
- N: 14.01 × 2 = 28.02
- O from NO₂: 16.00 × 4 = 64.00
- H: 1.008 × 6 = 6.048
- O from H₂O: 16.00 × 3 = 48.00
Total = 24.31 + 28.02 + 64.00 + 6.048 + 48.00 = 170.38 amu
✅ Final Answer:
- Formula: Mg(NO₂)₂·3H₂O
- Molecular Mass: 170.38 amu
