Calculating volume........(see below)?

Question

What volume (mL) of 0.139 M LiOH contains 25.0 g of LiOH ?

Answer 1

I'll tell you how to figure this one out...

Start by always converting to moles...
Convert 25g of LiOH to moles using its molecular weight.

Once you have that, you set up dimensional analysis to make your units cancel...

0.139M is the same as 0.139 mol/L

so... your answer above in mol * (L/0.139 mol) * (1000mL/L) = your final answer... the units all cancel and you are left with mL

Answer 2

First you need to convert the grams to moles. The molar mass of LiOH is equal to the molar masses of Li, O and H combined, or 6.941 + 15.999 + 1.007 = 23.947. So there are 23.947 grams in a mole and 1/23.947 moles in a gram. Multiply that by the number of grams to get the number of moles: 25/23.947 = 1.044 moles.

Now, the solution is 0.139 moles/liter, so it takes 1/0.139 liters to hold one mole. Multiply that by the number of moles needed: 1.044/0.139 = 7.511 L.

Answer 3

First change 25g of LiOH to moles using Molar Mass ratio:

25g x (1mol/23.95g) = A

Then, multiply that answer by the molarity conversion factor:

A x (1L / .139mol) = B

Then multiply that answer by 1000.

B x 1000 = Volume in mL.

This is just dimensional analysis. changing grams to moles by conversion, then moles to liters, then liters to milliliters.