Calculating Molarity Question

[akakakakak]

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Calculate the molarity of 9.55 molal solution of methanol (CH3OH) with a density of 0.937 g/mL.....

answer is 6.86M

...please and thank you 🙁

 

[jz123]

okay. so first the question asks you for the molarity of the methanol solution, which can be calculated by #moles of solute (MeOH) / Volume of solution(in L).
1st thing 1st, you need to realize that molality is #moles of solute/mass of solvent in kg. In this case, 9.55mol tells you that there are 9.55moles of solute (MeOH) in 1 kg of H2O (solvent). Next thing is that you have to figure out the total volume of the solution. you know the density of the solution is 0.937g/mL (d=mass/volume). Using the density equation can lead you to the volume of the solution if you know the mass of the solution. Here, the mass of MeOH is just 9.55moles * 32g/mole = 305.6g. and you have 1kg of water as the solvent. So total mass of the solution is 1305.6g. Then 1305.6g/ 0.937g/mL = 1393mL=1.393L.
Finally, 9.55mol/1.393L=6.86M.

 

[akakakakak]

thank sir

 

[Aamiracle]

For some reason, I'm not seeing how the mass jumped from 305.6g-->1305.6g


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[jz123]

when you add up the mass of water which is the solvent here and the mass of solute(MeOH), you will
get 1305.6g