Calculate the mass of Na2Cr2O7 to dissolve in 200g of water to prepare 1.5 m Na2Cr2O7(aq) solution.?

Answer 1

Are you trying to make a 1.5 molar solution (large M) or a 1.5 molal solution (which would be the small m)?

If it's molal, which is moles solute/kg solvent, you will need 0.3 moles of Na2Cr2O7, or 78.6 grams.

If it's molar, then you have to figure out if you want to end up with 200 mL of solution or you wish to add the sodium dichromate to 200 grams of water, which will result in more than 200 mL of solution. Given how it's written, however, I think you're looking for the molal concentration.

Note that I did that without being condescending, unlike some other people who are stuck at Level 2.

Answer 2

first of all it is 1.5 M with a capital M and that IS important

What does 1.5 M mean?
Well M means moles per liter.

So lets start backwards.

If we want moles per liter, but we only have 200g of water
Unless otherwise stated 1g of water is 1mL
So how many mL of water is that?
How many liters is that?

So if you multiply the volume of 200mL or 0.2L by the 1.5 Moles/Liter you get an anwer in moles of how many moles you need.

If you take that and the molecular weight then you should be able to determine the amount of grams.

Please try to understand why . .. not just put down the answer i romise you if you dont this exact problem is gonna show up on the exam and you are gonna feel real stupid when you miss it.

Answer 3

molality (m) = moles of solute/ solvent in Kg

moles of solute(Na2Cr2O7)= 1.5*0.2
=0.3 mole of Na2Cr2O7

Moles= wt. in gm/Molecular wt
Wt in gm= 0.3*260
= 78 gm Na2Cr2O7 required to make 1.5molal solution in 200gm(i.e 0.5 Kg) water.