Question

What will be the molality of the solution containing 18.25gof HCl gas in 500g of water? *

(a) 0.1m

(b) 1M

(c) 0.5m

(d) 1m

​

Answer 1

Given:-

→ Mass of HCl (solute) = 18.25g

→ Mass of water (solvent) = 500g

To find:-

→ Molality of the solution

Solution:-

• Molar mass of Hydrogen (H) = 1g/mol

• Molar mass of Chlorine(Cl) = 35.5g/mol

Hence, molar mass of HCl :-

= 1 + 35.5

= 36.5g/mol

Firstly, let's calculate the number of moles present in 18.25g of HCl :-

= Given Mass/Molar mass

= 18.25/36.5

= 0.5 mole

Now, let's convert the mass of water

from g to kg.

=> 1g = 0.001kg

=> 500g = 500(0.001)

=> 0.5kg

Molality of a solution :-

= Moles of solute/Mass of solvent in kg

= 0.5/0.5

= 1m

Thus, molality of the solution is

1m [Option.d]

Answer 2

$\large{\underline{\boxed{\boxed{\sf Let's \: Understand \: Concept \: F1^{st}}}}}$

Here, we have given that a solution contains 18.25g of HCl gas in 500g of Water and we, have to find it's molality. So, f1stly we will find No. of moles of HCl then, we will apply formula for molality and after substituting the values in the formula we will got our answer.

$\huge{\underline{\boxed{\sf Answer}}}$

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$\large{\underline{\sf Given:-}}$

❐Mass of HCl = 18.25g

❐Mass of water = 500g

$\large{\underline{\sf Find:-}}$

❐Molality of the solution

$\large{\underline{\sf Solution:-}}$

Here, we will find the no. of moles of HCl using:-

$\underline{\boxed{\sf No. \: of \: moles \: of \: HCl = \dfrac{Mass \: of \: HCl}{Molar \: Mass \: of \: HCl}}}$

where,

➡Mass of HCl = 18.25g

➡Molar Mass of HCl = Molar Mass of H + Molar Mass of Cl

= 1 + 35.5

= 36.5g

$\dashrightarrow\sf No. \: of \: moles \: of \: HCl = \dfrac{Mass \: of \: HCl}{Molar \: Mass \: of \: HCl}$

$\dashrightarrow\sf No. \: of \: moles \: of \: HCl = \dfrac{18.25}{36.5}$

$\dashrightarrow\sf No. \: of \: moles \: of \: HCl =0.5mole$

Now, using

$\underline{\boxed{\sf Molality = \dfrac{No. \: of \: moles}{ Mass \: of \: Solvent (in \: kg) }}}$

where,

• No. of moles = 0.5 mole
• Mass of Solvent = 500g = 500/1000 = 0.5kg

So,

$\dashrightarrow\sf Molality = \dfrac{No. \: of \: moles}{ Mass \: of \: Solvent (in \: kg)}$

$\dashrightarrow\sf Molality = \dfrac{0.5}{0.5}$

$\dashrightarrow\sf Molality = 1 mole/kg$

$\dashrightarrow\sf Molality = 1 m$

$\small{\therefore \underline{\sf Molality = 1 m}}$

Hence, Option (C) 1m will be correct option.