Question

Which of the following are pure substances?

Calcium chloride, used to melt ice on roads, consists of two elements, calcium and chlorine, in a fixed mass ratio.

Sulfur consists of sulfur atoms combined into octatomic molecules.

Cytosine, a component of DNA, consists of H, C, N, and O atoms bonded in a specific arrangement.

Baking powder, a leavening agent, contains 26% to 30% sodium hydrogen carbonate and 30% to 35% calcium dihydrogen phosphate by mass.

A rock is 10.6% by mass fayalite (Fe₂SiO₄), 15.7% by mass forsterite (Mg₂SiO₄), and the remainder silicon dioxide. What is the mass percent of each element in the rock?

% Fe

% Mg

% Si

% O

Answer 1

Let us consider 100 g rock sample.

so that,

mass of Fe2SiO4 = 10.6 g

mass of Fe present in sample = (10.6/203.77)*2*56 = 5.826 g

%Fe = 5.826%

mass of Mg2SiO4 = 15.7 g

mass of Mg present in sample = (15.7/203.77)*2*24 = 3.7

%Mg = 3.7%

% of SiO2 in sample = 100 - (10.6+15.7) =73.7 %

mass of Si in Sio2 = (73.7/60.1)*1*28 = 34.34 g

% of Si in the form of Sio2 = 34.34%

% of Si in the form of Fe2SiO4 = (10.6/203.77)*1*28 = 1.46%

% of Si in the form of Mg2SiO4 = (15.7/203.77)*1*28 = 2.16%

Total % of Si in sample = 34.34+1.46+2.16 = 37.96%

% of O in sample = (10.6/203.77)*4*16+(15.7/203.77)*4*16+(73.7/60.1)*2*16 = 47.5%