Example of Mass-Mass Problems

A balanced equation shows us the relationship between the masses of the products and the reactants.
How many moles of sodium chloride is needed to produce 355 g of chlorine?
NaCI ⇒ Na + Cl ₂
1. The equation is balanced: NaCI ⇒ 2Na + Cl ₂^
2. The molecular weight of each compound or element is calculated:
2 NaCl = 2 (23 + 35.5) = 2 (58.5) = 117 g
2 Na = 2X23 = 46g
C1₂ = 2 X 35.5 = 71 g
3. Information is read:
2 moles of NaCl (58.5 g), gives⇒ 2 moles of Na (46 g) + I mole of C1₂(71 g)
4. The requested data is written above the equation and the previous data is placed below:
Xg 355g
2NaCI ⇒ 2 Na + Cl ₂^
117g 46g 71g
A proportion is established ignoring Na, since it does not enter the problem:
Xg - 355g
117g - 71 g
The x is cleared:
X = (355gX117g) / 71g = 41535g / 71 = 585g
Result: 585 g of NaCl are needed to form 355 g of Cl ₂
Since the answer is asked in moles, divide the 585 g of NaCl by the molecular weight of a molecule of NaCl:
585 / 58.5 = 10 moles of NaCl