Question

An aqueous solution contains $30\%$ ( \[\dfrac{w}{v}\] ) of urea, what weight of urea will be present in 5 ml of solution?

Answer 1

Hint: The symbol \[\dfrac{w}{v}\] represents weight per volume, which means what amount of weight of a certain substance will be present in the solution. $30\%$ \[\dfrac{w}{v}\] urea means that there is 30 gram of urea present in 100 ml of solution.

Complete answer: Here, we have been given that an aqueous solution contains 30% \[\dfrac{w}{v}\] of urea, which means 30 gram of urea will be present in 100 ml of the solution.
So, if 100 ml of solution contains 30 gm of urea, how many grams of urea will be present in 5 ml of the solution. Thus, through simple cross multiplication, we can find out the weight of urea present in 5 ml of the solution.
Weight of urea present in 5 ml of solution is m,
 \[ \Rightarrow \] $m = \dfrac{5\times 30}{100}$ grams
Therefore, $m = 1.5 \;grams. $
Thus, in 5 ml of the solution, 1.5 grams of urea will be present.

Note:
Through this method, we can also find how much of urea will be present in any amount of solution. Just like weight per volume percentage is given in this question, weight per weight and volume per volume percentage are also there. In weight per weight, you can find out how much of the weight of a certain substance, and in volume per volume, you can find out the volume of certain substance in the solution.