Paradichlorobenzene, C6H4Cl2, is a component of mothballs. A solution of 2.00 g in 22.5 g of cyclohexane boils at 82.39 ∘C. The boiling point of pure cyclohexane is 80.70 ∘C. Calculate Kb for cyclohexane.

The Kb for cyclohexane is 2.79 °C/m

Explanation:

Step 1: Data given

Mass of Paradichlorobenzene = 2.00 grams

Mass of cyclohexane = 22.5 grams

Boiling point of the solution = 82.39 °C

Boiling point of pure cyclohexane = 80.70 °C

Molar mass of Paradichlorobenzene = 147 g/mol

Step 2: Calculate moles Paradichlorobenzene

Moles Paradichlorobenzene = mass / molar mass

Moles Paradichlorobenzene = 2.00 grams / 147 g/mol

Moles Paradichlorobenzene = 0.0136 moles

Step 3: Calculate molality

Molality = moles Paradichlorobenzene / mass cyclohexane

Molality = 0.0136 moles / 0.0225 kg

Molality = 0.605 molal

Step 4: Calculate Kb

Kb = change in boiling point / molality of solution  

 ⇒ Change in boiling point = 82.39 - 80.70 = 1.69 °C

⇒ molality = 0.605 molal

Kb = 1.69 °C / 0.605 molal = 2.79 °C/m

The Kb for cyclohexane is 2.79 °C/m

"among the transition metal elements, manganese and zinc have a slight lowering of electronegativities. the reason behind this is that the nuclear charges of these elements are slightly weaker than the other transition metal elements due to the way their valence electrons are filled up. both manganese and zinc have their valence electrons filled up to the 4th shell with 1 pair of electrons (4s2) completely occupying it. although the electrons in the 3rd shell (3d) are yet to be paired since the outer most shell is filled up, the ability of the nucleus to attract electrons is lessened. hence,   the electronegativity (or the ability to attract electrons) is lessened."

as given the equation is

2al + 6 hcl > 2 alcl3 + 3 h2(g)

as per equation two moles of aluminum reacts with six moles of hcl to give two moles of alcl3 and three moles of hydrogen gas

given: three moles of al are present and 7 moles of hcl

for three moles of aluminum nine moles of hcl will be required

however as 7 moles of hcl are present so the limiting reagent is hcl

hcl is going to decide the number of moles of product formed

when six moles of hcl are present = 3 moles of h2 are formed

when one mole of hcl is present == 3/6 mole sof h2 will be formed

when 7 moles of hcl are present = 3 x 7/ 6 moles of h2 will be formed = 3.5 moles of h2