Think of a relative mass as the “weight” of an atom compared to a standard unit – the hydrogen atom, which we set to 1.0. It’s like comparing the weight of a feather to a rock; the numbers tell you how many times heavier one is than the other. ⚛️
The relative mass of an element is simply the sum of the number of protons and neutrons in its nucleus.
Examples:
🔬
To find the relative mass of a molecule, add up the relative masses of all its atoms.
Example – Water (H2O):
\$\$
M{\text{H}2\text{O}} = 2 \times M{\text{H}} + M{\text{O}} = 2 \times 1 + 16 = 18
\$\$
So, one molecule of water has a relative mass of 18. 🧪
When two substances react, the masses that combine are in a simple proportion equal to the ratio of their relative masses.
General rule:
If substance A has relative mass \$MA\$ and substance B has relative mass \$MB\$, then
\$\$
\frac{\text{mass of A}}{\text{mass of B}} = \frac{MA}{MB}
\$\$
This is the same idea as mixing equal volumes of two liquids – the ratio of their “weights” tells you how much of each you need. 📐
| Element | Symbol | Relative Mass |
|---|---|---|
| Hydrogen | H | 1 |
| Carbon | C | 12 |
| Nitrogen | N | 14 |
| Oxygen | O | 16 |
| Sulphur | S | 32 |
Problem 1: 10 g of hydrogen reacts with oxygen to form water. How many grams of water are produced?
Solution:
\$\$
\frac{M{\text{H}2\text{O}}}{M{\text{H}2}} = \frac{18}{2} = 9
\$\$
So, for every 2 g of H₂ you get 18 g of H₂O.
\$\$
\text{Mass of H₂O} = 10\,\text{g} \times \frac{18}{2} = 90\,\text{g}
\$\$
📚
Problem 2: 12 g of carbon reacts with 24 g of oxygen. How much CO₂ is produced?
Solution:
\$\$
\frac{M{\text{CO}2}}{M_{\text{C}}} = \frac{44}{12} = 3.\overline{6}
\$\$
\$\$
\text{Mass of CO₂} = 12\,\text{g} \times \frac{44}{12} = 44\,\text{g}
\$\$
??
Problem 3: 5 g of nitrogen reacts with 20 g of hydrogen to form ammonia. What is the mass of NH₃ produced?
Solution:
\$\$
\frac{M{\text{NH}3}}{M_{\text{N}}} = \frac{17}{14} \approx 1.21
\$\$
\$\$
\text{Mass of NH}_3 = 5\,\text{g} \times 1.21 \approx 6.05\,\text{g}
\$\$
🎯