Moles, Mass, and Simple Calculations
Key definitions, formulas, and worked steps for basic stoichiometry.
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Questions Covered in This Set
10 cards to master
What is Avogadro's number and what does it represent?
6.02 × 10²³ — the number of particles in one mole of any substance.
State the master mole equation (and its rearrangements).
moles = mass ÷ molar mass (n = m/M); also m = n × M and M = m/n.
Why does the periodic table give molar masses?
The mole was defined so one mole of a substance weighs its relative atomic/formula mass in grams (e.g. C = 12 g/mol, H₂O = 18 g/mol).
How many moles are in 88 g of CO₂?
M(CO₂) = 12 + 2(16) = 44 g/mol, so n = 88/44 = 2.0 mol.
What mass is 0.25 mol of NaCl?
M = 23 + 35.5 = 58.5 g/mol, so m = 0.25 × 58.5 = 14.6 g.
What are the three steps of the reacting-mass method?
1) Convert known mass to moles (n = m/M); 2) use the mole ratio from the balanced equation; 3) convert moles back to mass (m = n × M).
6 g of Mg burns in 2Mg + O₂ → 2MgO. What mass of MgO forms?
n(Mg) = 6/24 = 0.25 mol; ratio 1:1 → 0.25 mol MgO; m = 0.25 × 40 = 10 g.
Formula for concentration of a solution?
concentration (mol/dm³) = moles ÷ volume (dm³); convert cm³ to dm³ by dividing by 1000.
What volume does one mole of any gas occupy at room temperature and pressure?
24 dm³ (24 000 cm³), so volume = moles × 24.
How is percentage yield calculated?
% yield = (actual mass ÷ theoretical mass) × 100 — e.g. 8.5 g collected of a predicted 10 g is 85%.
