Playing with Concentration 1

Chemistry Level 3

The molality of an alcohol $(\ce{C_2{H}_6O})$ solution is $3\text{ mol kg}^{-1}$ , and its density is $1.6\text{ g mL}^{-1}.$ What is the mass percentage of the solution?

If your answer is $20\ \%,$ write your answer as $0.2.$

Write your answer to 5 decimal places.

The answer is 0.12127.

2 solutions

Christopher Boo
Apr 11, 2014

The mass of $3\text{ mol }C_2H_6O$ is $3\times46=138\text{ g}$ .

Hence,

$\text{Mass of Percentage}$

$=\displaystyle \frac{\text{Mass of solute}}{\text{Mass of solution}}\times100\%$

$=\displaystyle \frac{138}{1138}\times100\%$

$=12.127\%$

The answer in decimal places will be $\boxed{0.12127}$

Note that if you want to use 5 significant figures in your answer 46 g/mol is inappropriate. Use 46.0684 g/mol instead.

Just trying to help.

Answer becomes 0.12142

Bernardo Sulzbach - 6 years, 11 months ago

How is mass of solution is 1138

Rifath Rahman - 6 years, 6 months ago

The Density thing fooled me :3

Mehul Arora - 5 years, 4 months ago

No, molarity is number of moles in 1 L of solution (not water).

$3 \space mol \space kg^{-1}$ means $3 \space mol$ in $1 \space kg$ of solution (not water).

I have made a report.

Chew-Seong Cheong - 5 years, 4 months ago

It is not good to ask for 5 decimal place for answer. Because the more accurate molar mass of $\ce{C2H6O}$ is 46.06844 g/mol. So if I use this figure instead of 46 g/mol I will get different answer.

Chew-Seong Cheong - 5 years, 4 months ago

$C:$ 12.0107

$H:$ 1.008

$O:$ 15.9994

$C_2H_6O:$ 46.0688

$\frac{3 \times 46.0688}{1000 + 3 \times 46.0688} \approx 0.1214247 \approx 0.12142$

Answer: $\boxed{0.12142}$

Lu Chee Ket - 5 years, 3 months ago

Yes you are right. $Mola\color{#D61F06}lity \equiv \dfrac{\text{Mass of solute}}{\text{Mass of solvent}}$ . Sorry, for the trouble.

Chew-Seong Cheong - 5 years, 3 months ago

Prince Loomba
Jan 29, 2016

We assume 1000 g water. Then 3mol=138 g C2H6O will be present (molality is moles of solute/mass of solvent.).Total mass of solution=1138 g and mass percentage/100=mass of C2H6O/mass of solution=138/1138=0.12127

No, molarity is number of moles in 1 L of solution (not water). $3 \space mol \space kg^{-1}$ means $3 \space mol$ in $1 \space kg$ of solution (not water). I have made a report.

Chew-Seong Cheong - 5 years, 4 months ago

Molality is defined as no. Of mol of solute upon wt of solvent not solution. You can check it out on web

Prince Loomba - 5 years, 3 months ago

Playing with Concentration 1

The answer is 0.12127.

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