Question

How do you find $0.1M$ of $CaS{O_4}$?

Answer 1

Molarity is defined as the number of moles of a compound present in $1000ml$ or $1l$ of a solution. It is a stoichiometric quantity representing the concentration in terms of number of particles present in the solution. $1$ Mole is defined as the amount which contains Avogadro’s number of particles i.e. $6.022 \times {10^{23}}$ particles.

Complete step-by-step answer:
Molar mass is a mass of $1$mole particles of the same type, it has the units of $g$(grams). Calcium sulphate (anhydrite) is the chemical name of $CaS{O_4}$ which represents the anhydrous state of Calcium sulphate.

To prepare $0.1M$of $CaS{O_4}$ we first need to need to know the solvent since it is not mentioned so the most obvious and random choice is ${H_2}O$(Water), Now since the amount of solution to be prepared is not mentioned so we take it equal to $1000ml$.

Next up we need to calculate the molar mass of $CaS{O_4}$ which is done by calculating the molecular mass of the compound which itself is obtained by using atomic masses of individual constituents, here they are $Ca$,$S$,$O$. Their corresponding atomic masses are $40,32,16$.

Now since the compound has $1Ca$,$1S$ and $4O$ so the combined molecular mass of $CaS{O_4}$ is : $40 + 16 + 4 \times 16 = 40 + 80 = 120$, therefore the molar mass of $CaS{O_4}$ is $120g$. To prepare $0.1M$ $1000ml$ solution we need to dissolve $0.1$moles of $CaSO4$ in $1000ml$of${H_2}O$, the mass corresponding to $0.1$moles is$120 \times 0.1 = 12g$. Thus on dissolving $12g$of$CaS{O_4}$ in $1000ml$ ${H_2}O$ we obtain the required solution.

Additional Information: Calcium sulphate naturally occurs in three forms CaS{O_4},CaS{O_4}.2{H_2}O,CaS{O_4}.{\raise0.5ex\hbox{\scriptstyle 1} \kern-0.1em/\kern-0.15em \lower0.25ex\hbox{\scriptstyle 2}}{H_2}Owhich are called Calcium sulphate Anhydrite, dihydrate and hemihydrate. Commonly named Anhydrite, Gypsum and Bassenite or plaster of Paris.

Note: When calculating molar mass be extra careful to take the correct values of atomic mass of atoms as in case of isotopes the atomic masses vary even though we have the same atomic number, also be careful when you add the atomic masses. Take a note of the amount of solution to be prepared as we need to modify the mass of the chemical to be added accordingly.