How we got there
Read this like a teacher at the board. We go slowly, name every number, and say why we take each step.
Explain in another language · 11
1. What is the question asking?
Read the question twice. Circle the unknown. Underline every number and unit. The paper is asking: A bag contains 80 g of ammonium nitrate, NH₄NO₃. M = 80 g·mol⁻¹ and each mole contains 28 g of nitrogen. What mass of nitrogen is in the bag?
Before any working — hunt first
Given
Numbers and units already in the sentence: 80 g · 80 g·mol⁻¹ · 28 g. Copy each one into a column. A name without a number is not given.
Missing
After you copy the given, the letter that still has no number is missing. The paper is pointing at: What mass of nitrogen is in the bag. Circle that word — that is the missing variable.
Find
We are required to find: What mass of nitrogen is in the bag. Write that as a symbol on the left, with a question mark, before you touch a formula.
Calculate
Write 8080=1 with empty letters. Cross out every letter that already has a number. Calculate the letter that is still standing — that gives What mass of nitrogen is in the bag.
In other words — as if you have never seen this before
If you have never seen this before: Find how many moles are in the bag, then multiply by the mass of nitrogen in one mole. We will name a method in words, write the formula with no numbers, then substitute one line at a time.
2. What do we already know?
m=80 g, M=80 g·mol⁻¹, nitrogen per mole=28 g.
3. Which method — and why this one?
Find how many moles are in the bag, then multiply by the mass of nitrogen in one mole.
4. Step-by-step working
01 / 05
Divide
n=8080=1 mol.
8080=1Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
02 / 05
Multiply
Nitrogen=1×28=28 g.
1×28=28Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
03 / 05
Sense-check
One mole of NH₄NO₃ contributes 28 g of N, not 2 g. The 14 is the molar mass of nitrogen.
28 gWhy this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
04 / 05
Step F — Box the answer with its unit
28 g of nitrogen
Why this step: A number without a unit is half a mark on a Physics paper, and a marker can miss a Maths answer that is buried in working.
05 / 05
Step H — What the marker is looking for
On an NSC memo you usually earn: (1) a method mark for naming the right formula or idea in words, (2) an accuracy mark for substituting the given numbers with units, (3) an accuracy mark for the boxed answer with its unit. Write those three lines even if you are unsure of the arithmetic — blank pages score zero.
Why this step: First-timers chase the final number and leave the method blank. The memo still pays out for the idea and the substitution even when the last line is wrong.
5. The answer
28 g of nitrogen
Copy onto your answer sheet — in this order
Formula: 8080=1
Answer: 28 g of nitrogen
6. Check it like a marker
Counting atoms and forgetting that each nitrogen atom has mass 14 gives a mass that is 14 times too small.
7. A trap first-timers fall into
First-timers often grab the first formula they remember, mix units (cm with m, minutes with seconds), or drop a minus sign. Write the formula in words first: “this equals that, because…”