Look at the following hypothetical reaction: a+3b--> c+3d assume that the equation is balanced. in which of the following reactant mole ratios will reactant b be the limiting reagent? a) a: 4b b) a: 5b c) 2a: 7b d) a: b

d) A:B

Explanation: Limiting reagent is the reagent which limits the formation of products.

Excess reagent is the reagent which is present in excess in the reaction.

a) A:4B

As seen from the given equation, 1 mole of A reacts with 3 moles of B, thus (4-3)= 1 mole of B will remain and thus A is the limiting reagent and B is the excess reagent.

b) A:5B

As seen from the given equation, 1 mole of A reacts with 3 moles of B, thus (5-3)= 2 moles of B will remain and thus A is the limiting reagent and B is the excess reagent.

c) 2A:7B

As seen from the given equation, 1 mole of A reacts with 3 moles of B, thus 2 moles of A will react with 6 moles of B. Thus (7-6)= 1 mole of B will remain and is the excess reagent.

d) A: B

As seen from the given equation, 3 moles of B react with 1 mole of A, thus 1 mole of B will react with 0.33 moles of A and (1 - 0.33) =0.66 moles of A will remain as excess reagent. B is the limiting reagent.

the missing options are :

(1) lower boiling point and a lower freezing point(2) lower boiling point and a higher freezing point(3) higher boiling point and a lower freezing point(4) higher boiling point and a higher freezing point

answer : the correct answer is option (3) higher boiling point and a lower freezing point

explanation :

when a non volatile solute is added to a solvent like water, it results in elevation of boiling point.

let us find the increase in boiling point of water when cacl₂ is added to water to make a solution of 1 m

step 1 : find molality of the solution

let us assume we have 1 l of solution.

let us convert this to grams of cacl₂.

molar mass of cacl2 is 111 g/mol

grams of cacl₂ =

mass of solute = 111 g

assuming density of solution same as water which is 1g/ml we have ,

mass of solution = density * volume

mass of solution =

mass of solution = mass of solvent + mass of solute

mass of solute = 1000g - 111g

mass of solute 889 g = 0.889 kg

the molality of the solution can be calculated as

molality = 1.12 m

step 2 : find boiling point elevation.

the elevation in boiling point is calculated as

here δtb = boiling point elevation

i = vant hoff factor which is the number of ions formed by the solute .

cacl₂ dissociates as

it forms 3 ions . therefore,

i = 3

m = molality which is 1.12 m

kb is constant for a given solvent. for water it is 0.512

the boiling point of water increases by 1.7 °c

step 3: find freezing point depression.

when a non volatile solute is added to water, it results in decrease in freezing point.

the formula to find freezing point depression is

kf is constant for water and its value is 1.86

the freezing point of water decreases by 6.2 °c

therefore we can say that, the solution will have a higher boiling point and a lower freezing point