Convert an amount of substance between moles, millimoles and micromoles.
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250 mmol = 250 × 0.001 ÷ 1 = 0.25 mol.
How many water molecules are there in 36.0 g of water?
Step 1 — molar mass of H₂O:
H: 2 × 1.008 = 2.016 · O: 1 × 15.999 = 15.999 · M = 18.015 ≈ 18.02 g/mol
Step 2 — moles:
n = 36.0 ÷ 18.02 = 2.00 mol
Step 3 — molecules:
N = 2.00 × 6.022 × 10²³ = 1.204 × 10²⁴ molecules
Bonus: each H₂O molecule contains 3 atoms, so the sample holds 3 × 1.204 × 10²⁴ = 3.613 × 10²⁴ atoms. Read the question carefully — "molecules" and "atoms" are different answers.
Quoted from The Mole Concept — Why 6.022 × 10²³ and How to Use It.
What is the mass of 0.25 mol of calcium carbonate, CaCO₃?
Molar mass:
Ca: 1 × 40.078 = 40.078 · C: 1 × 12.011 = 12.011 · O: 3 × 15.999 = 47.997
M = 40.078 + 12.011 + 47.997 = 100.086 ≈ 100.09 g/mol
m = n × M = 0.25 × 100.09 = 25.02 g
Quoted from The Mole Concept — Why 6.022 × 10²³ and How to Use It.
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