Michael S. answered • 14d
B.S. in Chemistry, Indiana University; Organic Chem Teaching Intern
50.2 g KBr
Two conversions, in this order: volume → moles → grams.
Step 1 — convert the volume to litres
Molarity is defined as moles per litre, so millilitres have to go first:
485 mL × (1 L / 1000 mL) = 0.485 L
This is the step that gets skipped. Multiplying 485 by 0.870 gives 422 mol, which would be roughly 50 kilograms of KBr in a beaker — a good reason to sanity-check the magnitude of your answer.
Step 2 — moles of solute
n = M × V = (0.870 mol/L)(0.485 L) = 0.422 mol KBr
Step 3 — molar mass of KBr
K: 39.10 g/mol + Br: 79.90 g/mol = 119.00 g/mol
Step 4 — convert to grams
m = n × M = (0.422 mol)(119.00 g/mol) = 50.2 g
A quick mental check
You have roughly half a litre of a solution that is a bit under 1 M, so you expect a bit under half a mole. Half a mole of something weighing ~119 g/mol is around 60 g, and "a bit under" lands you near 50. That estimate takes five seconds and catches any factor-of-1000 error immediately.
One distinction worth being clear on
Molarity is moles of solute per litre of solution, not per litre of solvent. So the 485 mL is the total volume of the KBr solution, not the water it was made with. It matters when you prepare one: you dissolve the solid first, then add water up to the mark — you do not add solid to 485 mL of water, since dissolving changes the volume.
The same three steps run in reverse if a problem asks what volume of 0.870 M KBr contains 50.2 g: convert grams to moles with the molar mass, then divide by molarity to get litres. Every solution-stoichiometry problem is some path through grams ⇆ moles ⇆ litres, with molar mass and molarity as the two bridges.
