Question

What is the molarity of a 10ml solution in which 3.7g KCl is dissolved?
(A) $0.05M$
(B) $0.1M$
(C) $1M$
(D) $5M$
(E) $10M$

Answer 1

As we have learnt that Molarity is basically the amount of a substance dissolved in a certain volume of solution. It is also defined as the number of moles of a solute dissolved in 1 litre of solution.

Complete step by step solution:
So we know that Molarity is basically the amount of a substance dissolved in a certain volume of solution. It is also defined as the number of moles of a solute dissolved in 1 litre of solution or we can say that molarity is the concentration of a substance or particularly a solute in a given volume of solution. The Molarity has the units as: $mol{L^{ - 1}}\;or\;M\;or\;molar$. We can easily calculate molarity of any substance by using formula:
$Molarity = \dfrac{{moles\;of\;solute}}{{volume\;of\;solution\;(L)}} \\\ or \\\ Molarity = \dfrac{{weight\;of\;solute \times 1000}}{{molecular\;weight\;of\;solute \times Volume\;of\;solution\;(ml)}} \\\$
As moles of a solute is the ratio of its given weight and molecular weight, we can replace the moles with this ratio as we already have the weight of solute.
We are given with:
$vol.\;of\;solution = 10ml$
$weight\;of\;KCl = 3.7g$ and we also know the $molecular\;weight\;of\;KCl = 74.55g$
Using the formula we can calculate molarity:
$Molarity = \dfrac{{weight\;of\;solute \times 1000}}{{molecular\;weight\;of\;solute \times Volume\;of\; solution\;(ml)}} \\\ M = \dfrac{{3.7 \times 1000}}{{75.55 \times 10}} \\\ M = 5mol{L^{ - 1}} \\\$
Therefore the correct answer is (D).

Additional information: Like molarity we can also find the molality of any substance which is defined as amount of solute present in $1kg$ of solvent or the number of moles of solute dissolved in $1kg$ of solvent.
Using molarity we can calculate normality as it is defined as the number of mole equivalents dissolved in $1L$ of solution. So when molarity is given we can use $N = n \times M$where n is any integer. It is also interesting to know that normality of a solution is never less than its molarity.

Note: Molality is independent of temperature since mass remains unaffected with temperature whereas Molarity, Normality and Formality of any substance is dependent on temperature as the temperature increases, number of particles in a given volume decreases and thus molarity increases but number of moles remains unchanged.