ACT Science: Molar Mass and Stoichiometry Fundamentals

Molar mass is the mass (in grams) of one mole of a substance. It's found on the periodic table as the atomic mass. For example, carbon=12, hydrogen=1, oxygen=16. To find molar mass of CO2: 12+(16×2)=44 g/mol. This number is your conversion factor. If a question gives you "44 grams of CO2," you convert to moles: 44 g÷44 g/mol=1 mole. If it gives you 0.5 moles, you convert to grams: 0.5 mol×44 g/mol=22 grams. Every stoichiometry problem starts with converting between grams and moles using molar mass.

Example: A reaction consumes 18 grams of water. Water's molar mass=18 g/mol (2+16). So 18 g÷18 g/mol=1 mole of water. On the ACT, you'll see this pattern repeatedly. Build the habit: calculate molar mass, then divide (or multiply) to convert.

Error 1: Forgetting to add up all atoms in the molecule. For H2SO4, it's 2(1)+32+4(16)=98, not 2+32+4=38. Error 2: Using the wrong direction in the conversion (dividing when you should multiply). Error 3: Dropping units, so you lose track of whether you need g/mol or mol/g. Error 4: Rounding too early in multi-step problems. Error 5: Forgetting that the periodic table gives atomic mass, not molecular mass for elements. Write units on every number: "44 g CO2" not just "44," so you never confuse which conversion to use.

Checklist: (1) Write out the molecule's atoms. (2) Look up each atom's mass. (3) Multiply and add. (4) Double-check your arithmetic. (5) Write the conversion equation (grams÷molar mass=moles). (6) Plug in numbers with units.

Problem 1: What is the molar mass of NaCl? (Na=23, Cl=35.5). Answer: 23+35.5=58.5 g/mol. Problem 2: How many moles are in 29.25 grams of NaCl? Answer: 29.25 g÷58.5 g/mol=0.5 mol. Problem 3: Convert 2 moles of O2 to grams. (O=16). Answer: 2 mol×32 g/mol=64 grams. After each problem, write the conversion factor you used in a box: "÷molar mass to get moles" or "×molar mass to get grams."

Drill: Calculate molar mass for CO, CH4, and H2O. Then convert 88 grams of CO, 16 grams of CH4, and 18 grams of H2O to moles. Answers: 28 g/mol→3.14 mol; 16 g/mol→1 mol; 18 g/mol→1 mol. Do five more on your own daily.

About 1-2 chemistry questions per ACT Science section involve stoichiometry or molar mass calculations. These are almost always solvable if you know the algorithm; there's no trick or deep chemistry needed. Many students skip these questions because the chemistry sounds scary, but it's really just unit conversion. Once you can convert grams to moles, you can handle any stoichiometry problem the ACT throws at you, unlocking free points others miss.

Spend one week drilling molar mass calculations. By test day, it should be automatic. That confidence transfers to your overall Science score and gives you time to focus on harder passages.