An aqueous solution of 2.7 M hydrochloric acid has a density of 1.1 g/mL. How many mmol of hydrochloric acid are present in 15.4 mL of this solution?

Answer 1

2.7 M is defined as 2.7 moles HCl/liter of solution = 2.7 moles/1000 ml solution

Using dimensional analysis we can find moles in 15.4 ml solution....

2.7 mol HCl/1000 ml x 15.4 ml = 0.04158 moles in 15.4 ml

Converting to mmoles...

0.04158 moles x 1000 mmol/mol = 41.58 mmoles

Using correct number of significant figures gives us an answer of 42 mmoles

(NOTE: no need to use the density of the solution)

Answer 2

The density is irrelevant (you would need it if you started with molality). Here, you start with ml of solution, convert to liters of solution, use the molarity to find moles of HCl, convert to mmoles of HCl.