Question

Given, 15.5g of potassium nitrate is dissolved in 200$c{{m}^{3}}$ of water. How many moles of potassium nitrate are present in 15.5g of the compound?

Answer 1

By the knowing molar mass of potassium nitrate and then finding the number of moles of it and then find out the total weight of the compound using the density of water and also the volume of water.

Complete step by step solution:
In the previous classes, we have studied the basic topic such as the conversion factors from moles to grams, grams to kilograms and so on.
Let us now see the moles of the compound potassium nitrate present in 15.5g of the compound.
Firstly let us calculate the molar mass of the compound that is of $KN{{O}_{3}}$
It is calculated as, $39.1+1\times 14+3\times 16=101.1g/mol$
Let us now convert these grams into moles and this is is shown below,
$\dfrac{15.5g}{101.1g/mol}=0.1533mol$
Now, let us assume the density of water to be as 1 g/L
Thus, total weight of the compound will be $=15.5+200=315.5g$
Now, to find the total moles of potassium we shall take the ratio of 0.1533 mol of $K$ that are dissolved to the 315.5g of the compound which is multiplied by 15g of the compound.
This will be, $\dfrac{0.1533}{315.5}\times 15=0.0075mol$ of $KN{{O}_{3}}$
Therefore, the correct answer is 0.0075 moles of potassium nitrate are present in 15.5g of the compound.

Note:
It is important to note that the number of moles of any compound can be sound by using the data of the amount of the substance given and the molar mass of that compound and these values are divided. That is the mass of the compound is divided by the molar mass and this gives expression in grams.