Chemistry / Unit 3 Chem 0403 worked example, 60 to 90 seconds
Calculating Solute Mass from Molarity and Volume
The idea
Calculating a solute's mass from molarity and volume means multiplying molarity by volume in liters to get moles of solute, then multiplying that mole amount by molar mass to get a mass in grams.
In practice
A chemist needs the mass of NaCl dissolved in 0.250 L of a 5.30-M solution. Multiplying 5.30 mol/L by 0.250 L gives 1.325 mol of NaCl present in that volume of solution. Multiplying 1.325 mol by NaCl's molar mass, 58.44 g/mol, gives about 77.4 g of NaCl, the mass that must have been weighed out to prepare this particular solution from scratch.
Where people slip
Rounding the intermediate mole value before the final multiplication, rather than carrying a guard digit, can shift the final mass by several tenths of a gram.
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