Chemistry, 10.07.2019 03:30 hsbhxsb

A0.2436-g sample of an unknown substance was dissolved in 20.0 ml of cyclohexane. the density of cyclohexane is 0.779 g/ml. the freezing-point depression was 2.5ºc. calculate the molar mass of the unknown substance

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Answer from: babygirl10302015

The molar mass of the unknown substance is 125 g/mol.

What is cyclohexane?

Cyclohexane is a colorless, flammable liquid which has a detergent like odor.

It is a cycloalkane whose formula i s $\bold{C_6H_1_2}$ .

Given,

Density is 0.779 g/ml

The freezing-point depression is 2.5ºc.

Mass is 0.2436-g

In kg = 0.0002436 kg

Now,

The depression in freezing point is equal to the molality of solution which is

$\bold{\Delta T_f= k_fm}$

where $\bold{ k_f}$ is equals to freezing point depression constant, which is equal to 20.0 ml.

Step 1: Calculating the molality

$\bold{m= \dfrac{\Delta T_f}{k_f} = \dfrac{2.5^\circ c}{20^\circ c\;kg/mol}=0.125\;mol/kg }$

Step 2: Calculating the Mass of cyclohexane

$\bold{m= d\times V= 0.779\;g/ml \times20\;ml =15.58\;g}$

Converting mass into kg

$\bold{1\;g= 10^-^3 kg}$

15.58 g = 0.01558 kg

Step 3: Calculating the number of moles

$\bold{n = \dfrac{mass}{molar\;mass} = \dfrac{0.0002436\;kg}{M} }$

Step 4: Expression for the molality of the solution

$\begin{aligned}\bold{m &= \dfrac{n_s_o_l_u_t_e}{m_s_o_l_u_t_e} \\\\0.125\;mol/kg & = \dfrac{0.015635}{M} \\\\M &= \dfrac{0.015635}{0.125\;mol/kg} = 0.1250\;kg/mol\\\\M &= 0.125\;kg/mol \dfrac{1000\;g}{1\;kg} = 125\;g/mol }\end{aligned}$

Thus, the molar mass of the substance is 125 g/ mol.

Learn more about cyclohexanes, here:

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Answer from: skrillex88

The depression in freezing point is related to molality of solution as follows:

$\Delta T_{f}=k_{f}m$

Here, $k_{f}$ is freezing point depression constant, for cyclohexane it is equal to 20°C kg/mol.

The value of freezing-point depression is 2.5 °C, molality can be calculated as follows:

$m=\frac{\Delta T_{f}}{k_{f}}=\frac{2.5 ^{o}C}{20 ^{o}C kg/mol}=0.125 mol/kg$

Molality is defined as number of moles of solute in 1 kg of solvent.

Here, solvent is cyclohexane, its volume is given 20.0 mL and density is 0.779 g/mol thus, mass of cyclohexane can be calculated as follows:

$m=d\times V=0.779 g/mL\times 20 mL=15.58 g$

Converting this into kg,

$1 g=10^{-3} kg$

Thus, 15.58 g will be 0.01558 kg.

Now, number of moles of unknown solute is related to its mass and molar mass as follows:

$n=\frac{m}{M}$

Putting the values of mass of solute which is 0.2436 or 0.0002436 kg

$n=\frac{ 0.0002436 kg}{M}$

Now, expression for molality of solution is:

$m=\frac{n_{solute}}{m_{solvent}}$

Putting all the values,

$0.125 mol/kg=\frac{0.0002436 kg}{M\times 0.01558 kg}$

Or,

$0.125 mol/kg=\frac{0.015635}{M}$

On rearranging,

$M=\frac{0.015635}{0.125 mol/kg}=0.1250 kg/mol$

Or,

$M=0.125 kg/mol(\frac{1000 g}{1 kg})=125 g/mol$

Therefore, molar mass of unknown sample is 125 g/mol

Answer from: Quest

The correct answer is "atomic weight".

Answer from: Quest

hydrocarbons are organic compounds that contain only hydrogens and carbons. they can be saturated (those which contain only carbon-carbon single bonds) or unsaturated (those which contain carbon-carbon double or triple bonds) in nature. hydrocarbons can undergo several reactions like substitution, elimination etc.

on substituting one or more hydrogen atoms in hydrocarbon it results in the formation of haloalkane. as the halogen atoms are large compared to the carbon and hydrogen atoms, due to which the molecular weight increases and the bond become polar because of presence electronegative halogen atom and results in the increase in boiling point of the haloalkane.

thus, the correct option is (b) that is the boiling point of the new compound increases on substituting a hydrogen atom with a halogen in a hydrocarbon.

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