Question

To prepare a solution that is $0.50M$ $KCl$ starting with $100ml$ of $0.40M$ $KCl$ need to:
A.Add $0.75g$ $KCl$
B.Add $20ml$of water
C.Add $0.10mol$ $KCl$
D.Evaporate $10ml$ water

Answer 1

Hint: At first we will write the things given in the question. Then we will calculate the Moles of $KCl$ . Now to make the $0.50M$ $KCl$ we will need more moles of the $KCl$ . Then we will calculate the number of moles of $KCl$ to be added into the solution. From those moles we will calculate the mass of $KCl$ we need to add to the solution we are preparing.

Complete step by step answer:
Step1.
The molarity of the original solution is $0.40M$ .
The volume of the original solution is $100ml$ .
The molarity we need is $0.50M$
Step2.
We will calculate the moles of $KCl$ solution with $0.40M$.
$Moles = \dfrac{{volume}}{{1000ml}} \times Molarity$
$Moles = \dfrac{{100ml}}{{1000ml}} \times 0.40M$
$Moles = 0.040$
Step3. Now we will calculate the number of moles of $KCl$ required to make it $0.50M$ with $100ml$ of the volume.
$ \Rightarrow \dfrac{{100ml}}{{1000ml}} \times 0.50 = 0.050$
So we need $0.050$ moles of the$KCl$ to make it a $0.50M$ solution.
Step4. So now we will subtract the number of moles given from the number of moles given.
$ \Rightarrow 0.050 - 0.040 = 0.010$
We need $0.010$ moles need to be added to $KCl$ in the solution.
Step5. Now we will calculate the mass of $KCl$
$KCl = 39 + 35.5 = 74.5$
So the total mass needed to be added will be moles multiplied by the given mass.
$ \Rightarrow 0.010 \times 74.5 = 0.75$ approx.
So we need to add $0.75g$ of the $KCl$ to the solution
Hence option A is the correct answer.

Note:
Molarity is the way to measure the amount of the object dissolved in the fixed volume of the solution. It is defined as a moles of solute per liter of solution. It is also called the molar concentration of a solution. The unit of Molarity is ‘M’.