Answer:
2.05 moles of SiO₂
Explanation:
1- The molar mass of the compound must be calculated. To do this, with the help of a periodic table the atomic weights of the elements of the compound are sought and the sum of them is performed:
mSiO₂= mSi + 2 x mO = 28.09 g + 2 x 15.99 g = 60.07 g / mol
2- Once the molar mass is obtained, you can calculate the number of moles that represent the amount of SiO₂ in the statement. Since the data is in units of kilograms, and the molecular mass that has been calculated is in grams, a unit conversion of some of the two data must be performed, to have them in the same unit. All data will be passed to grams.
So:
1 kg ____ 1000 g
0.123 kg ____ X = 123 g
Calculation: (0.123 kg x 1000g) / 1 kg = 123 g of sample
3- Now the mass of SiO₂ can be converted to moles:
60.07 g SiO₂ _____ 1 mol
123 g SiO₂ ____ X = 2.0476 mol
Calculation: (123 g x 1 mol) / 60.07 g = 2.0476 mol ≅ 2.05 mol
As the statement says that we use three significant figures, only three figures of the calculated result are left and according to the fourth figure obtained that is a 7, we round the last significant figure by adding a unit.
As a result, 0.123 kg of SiO₂ equivalent to 2.05 moles.
