How many atoms are in 0.500 g of Magnesium Chloride?

3.16 × 10²¹ atoms MgCl₂

General Formulas and Concepts:

Explanation:

Step 1: Define

0.500 g MgCl₂ (Magnesium Chloride)

Step 2: Define conversion

Molar Mass of Mg - 24.31 g/mol

Molar Mass of Cl - 35.45 g/mol

Molar Mass of MgCl₂ - 24.31 + 2(35.45) = 95.21 g/mol

Avagadro's Number: atoms

Step 3: Dimensional Analysis

= 3.16248 × 10²¹ atoms MgCl₂

Step 4: Check

We are given 3 sig figs. Follow sig fig rules.

3.16248 × 10²¹ atoms MgCl₂ ≈ 3.16 × 10²¹ atoms MgCl₂

n2(g) + 3h2-> 2nh3(g)   this is the balanced equation

note the mole ratio between n2, h2 and nh3.   it is 1 : 3 : 2   this will be important.

moles n2 present = 28.0 g n2 x 1 mole n2/28 g = 1 mole n2 present

moles h2 present = 25.0 g h2 x 1 mole h2/2 g = 12.5 moles h2 present

based on mole ratio, n2 is limiting in this situation because there is more than enough h2 but not enough n2.

moles nh3 that can be produced = 1 mole n2 x 2 moles nh3/mole n2 = 2 moles nh3 can be produced

grams of nh3 that can be produced = 2 moles nh3 x 17 g/mole = 34 grams of nh3 can be produced

note:   the key to this problem is recognizing that n2 is limiting, and therefore limits how much nh3 can be produced.

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