Question

A 4-g mass of NaOH is dissolved in 5mL of water.
what is the approximate molar concentration of the NaOH?

Answer 1

4g/(40g/mol) / 5mL = 0.1mol/5mL

0.1mol/5mL = x mol/1000mL

x = 0.1mol/5mL * (1000mL/5mL) = 2 M

Answer 2

why is 4g/40g/mol?

Answer 3

NaOH is 40g per mol

Answer 4

oh okay

Answer 5

you want to know much mols of NaOH is contained in 4g

Answer 6

alright

Answer 7

remember to work out the concentration of a solution you want to work out how much solute is dissolved in your solution. So if you were trying to make a solution of sugared water, sugar would be the solute and water would be solvent.

In your case you have Sodium Hydroxide dissolved in water. When doing these sorts of calculations you must use the standard SI units. For volume you should use 1 Litre (1000 ml) and for mass it's grams. Convert your units then use the formulas

concentration = mass/volume
moles = mass/molar mass