HS-PS1-7: Using Moles to Show Mass Is Conserved
Also called NGSS HS-PS1-7 · Topic: Chemical Reactions · Physical Science
The Official Wording
Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
Clarification statement: Emphasis is on using mathematical ideas to communicate the proportional relationships between masses of atoms in the reactants and the products, and the translation of these relationships to the macroscopic scale using the mole as the conversion from the atomic to the macroscopic scale. Emphasis is on assessing students’ use of mathematical thinking and not on memorization and rote application of problem-solving techniques.
Assessment boundary: Assessment does not include complex chemical reactions.
NGSS Lead States, 2013 · Read it on nextgenscience.org · checked October 15, 2026
What HS-PS1-7 Means in Plain English
High school students use math to argue that a reaction rearranges atoms without creating or destroying any, so total mass stays the same. They balance equations, use the mole to move between atom counts and grams, and compare reactant and product masses.
Reactions stay simple, and the focus is reasoning with proportions rather than memorized procedures. It deepens the middle school particle model of conserved mass (MS-PS1-5) and supports quantitative carbon cycle models in HS-ESS2-6.
HS-PS1-7 I Can Statements
- I can balance a simple equation so atoms match on both sides.
- I can convert between grams and moles using molar mass.
- I can show with numbers that mass is conserved in a reaction.
HS-PS1-7 Worked Example
A problem worked from real lab amounts, the kind students meet when they heat iron filings with sulfur powder.
A student heats 5.6 g of iron filings with 3.2 g of sulfur powder, and both react completely. Use moles to predict the mass of iron(II) sulfide produced.
- Write and balance the equation, counting atoms on each side.Fe + S → FeS: 1 Fe and 1 S on each side
- Convert the iron to moles with its molar mass of 55.8 g/mol.5.6 g ÷ 55.8 g/mol ≈ 0.10 mol Fe
- Convert the sulfur to moles with its molar mass of 32.1 g/mol.3.2 g ÷ 32.1 g/mol ≈ 0.10 mol S
- Use the one-to-one ratio to find moles of product.0.10 mol FeS
- Convert to grams with the molar mass of FeS, 87.9 g/mol.0.10 mol × 87.9 g/mol ≈ 8.8 g
Check it: The reactants total 5.6 g + 3.2 g = 8.8 g, the same as the product, because every iron and sulfur atom ends up in the FeS.
Common Mistakes with HS-PS1-7
Comparing grams instead of moles
“There is more iron than sulfur, so sulfur runs out first.”
Try this: Reactions combine particles, not grams. Convert both masses to moles first, then compare them using the ratio from the balanced equation.
Balancing by changing subscripts
Turning H₂O into H₂O₂ to fix the oxygen count
Try this: Change only the coefficients in front of formulas. Changing a subscript turns the substance into a different compound entirely.
Ignoring gases in an open container
“The burned wood weighs less, so mass was lost.”
Try this: Gases like carbon dioxide and water vapor escaped into the air. Count their mass too and the totals match.
Where HS-PS1-7 Fits
Practice HS-PS1-7
Quick Check
- How many oxygen atoms are on each side of 2H₂ + O₂ → 2H₂O?
- What is the mass of 2.0 mol of water, with a molar mass of 18.0 g/mol?
- Burning 10 g of magnesium in a sealed flask gives a product heavier than 10 g. Was mass created?