Question

: What is the number of moles of substance per liter of solution?
1) Molarity
2) Molality
3) Mole fraction
4) Density
5) Partial pressure

Answer 1

Hint: Molarity is defined as the ratio of the number of moles of solute to the volume of the solution. It is also generally known as concentration.

Complete answer:
Molarity denotes the number of moles of solute per liter of the solution. Its unit is $mol/L$or $mol/d{m^3}$. It is the most common term used to determine the concentration of a solution.
Molarity is used to determine the amount of solute or the volume of the solvent in the solution.
If we are given two solutions, having same amount of moles of solute, it can be given by this relation: ${c_1}{V_1} = {c_2}{V_2}$
Where, c is the concentration and V is the volume. For calculating the molarity, we need to find the number of moles. The number of moles is the ratio of the amount of the solute (in grams) to the molar mass of the substance. It can be given as:
$no.of{\text{ moles}} = \dfrac{{amount{\text{ of solute }}(g)}}{{Molar{\text{ mass}}(g/mol)}}$
The formula for the molarity can be given as:
$Molarity = \dfrac{{moles{\text{ of solute(mol)}}}}{{Volume{\text{ of solution}}(L/d{m^3})}}$
Hence the correct answer is option A. Molarity.
Option B is incorrect because molality is the ratio of no. of moles of solute to the mass of the solvent (in kg). The formula for molality can be given as:
$molality = \dfrac{{moles(solute)}}{{mass{\text{ of solvent }}(kg)}}$
Option B is wrong.
Mole fraction is the ratio of no. of moles of solute/solvent to the total no. of the moles of solute and solvent. Hence option C is wrong.
Density is the ratio of mass to the volume of the solution. Option D is incorrect.
Partial pressure is the difference between the pressure of the solution to the pressure of the pure solvent. Option E is incorrect.

Hence the correct answer is option A. Molarity.

Note:
Remember that in Molarity the volume of the solution should always be expressed in litres or $d{m^3}$. Remember the following conversions:
$
  1L = 1d{m^3} \\
  1d{m^3} = 1000c{m^3} = 1000mL \\
$
The amount of solute for the calculation of no. of moles should always be in grams.
$
  1g = {10^{ - 3}}kg \\
  1g = 1000mg \\
$
Take care of the calculations while doing such numericals.