figure out how many moles of NaOH were added to the flask. start with 40.05 mL. Remember that 0.250 M means 0.250 moles / 1L

Answer 1

Always start with what you have. Here, that is 40.05mL of solution. You will notice that molarity units are given in moles/L, so we will need to convert our 40.05mL to L.

There are 0.001 liters in 1 milliliter or 1000 milliliters in one liter. To convert from milliliters to liters, multiply 40.05 by 0.001 (or divide by 1000). This gives us .04005L.

So M= moles/L. We know M (.250) and we know our volume (.04005L). We need to find moles. We can plug these into this equation for molarity:

.250M = x moles

.04005L

Multiply both sides by .04005 L. This gets rid of this value on the right side of the equation. The liter units cancel out on the left and we get .0100125 moles = x. Round to .01 moles of NaOH for significant figures.

Another way to look at this, which works better for some students, is to do it as a straight conversion problem. You start with your .04005L and convert it to moles using the molarity to cancel out the liter units as below:

.04005L x .250mol = .0100125 moles NaOH or .01 moles NaOH

1L

I hope this helps!