Michael S. answered • 19d
B.S. in Chemistry, Indiana University; Organic Chem Teaching Intern
6.09 g, 0.152 mol
This row runs the opposite direction from the MgSO4 row. There you were given a mass and worked toward molarity; here you are given the molarity and work back to mass. Same two relationships, reversed.
Step 1 — moles, from molarity and volume
Molarity is moles per litre, so multiplying by the volume in litres returns moles:
105.0 mL = 0.1050 L
n = MV = (1.45 mol/L)(0.1050 L) = 0.152 mol NaOH
Step 2 — mass, from moles and molar mass
M(NaOH) = 22.99 (Na) + 16.00 (O) + 1.01 (H) = 40.00 g/mol
m = nM = (0.152 mol)(40.00 g/mol) = 6.09 g
Enter it in the order the box asks for: mass first, then moles — 6.09, 0.152. The units printed after the blank ("g, mol") confirm the order, and it is the reverse of the sequence you calculated in. Worth double-checking, because it is an easy place to transpose two correct numbers into a wrong answer.
Why this row is the easiest one on the table
NaOH has a molar mass of almost exactly 40 g/mol, which makes the arithmetic checkable in your head: 0.152 × 40 is a little over 6. If your answer had come out near 0.6 or 60, a factor-of-ten slip in the volume conversion is the first thing to look at.
The general shape
Every blank in this table is one of these two equations rearranged:
n = m / M(molar mass) and M(molarity) = n / V
Moles appear in both, so moles is always the bridge. Whichever quantity you are handed, convert to moles first and the remaining blank is a single step away — and volume always goes in as litres.
