Can someone please explain to me why we don’t convert to moles. I don’t understa
ID: 706283 • Letter: C
Question
Can someone please explain to me why we don’t convert to moles. I don’t understand when you can just use the moles in the balanced equation versus when you have to calculate moles. I thought for this question you would have to calculate moles. Can someone explain? 3.62 Magnesium metal burns in oxygen to form magnesium oxide, Mgo. (a) Write a balanced equation for the reaction. (b) How many grams of oxygen are needed to react with 25.0 g of Mg? How many grams of MgO will result? (c) How many grams of Mg are needed to react with 25.0 g of O,? How many grams of MgO will result?
Explanation / Answer
We can convert it to moles and it make calculations easier.It can be done by the following way:
a) 2Mg + O2 = 2MgO
b) molar mass of Mg = 24.3 g / mol
Weight of Mg = 25 g
Moles of Mg = weight / molar mass = 25 / 24.3 = 1.03 mole
Now from the reaction it is clear that:
2 moles of Mg reacts with 1 mole of Oxygen
So, 1 mole of Mg reacts with 0.5 mole of Oxygen
Hence , 1.03 mole of Mg will react with 0.5× 1.03 = 0.52 mole of oxygen
Now weight of oxygen in grams :
Mole of oxygen = weight in gram / molecular mass
Molecular mass of Oxygen = 2 × 16 = 32
0.52 × 32 = weight in gram = 16.46 g of oxygen
Now mass of MgO
2 mole of Mg yields 2 mole of MgO
1.03 mole of Mg will yield 1.03 mole of MgO
Weight of MgO in gram
Mole = weight in gram / molecular mass
Weight in gram = 1.03 × 40.51 = 41.51 g
c) According to the reaction :
Moles of Oxygen = 25 / 32 = 0.78 mole
1 mole of Oxygen reacts with 2 mole of Mg
0.78 moles will react with = 2× 0.78 = 1.56 mole of Mg
Moles = weight in gram / molecular mass
Weight in gram = 1.56 × 24.3051 = 38 g Mg
Mass of MgO:
1 mole of Oxygen yields 2 mol MgO
So; 0.78 mole will yield 0.78 × 2 = 1.56 mole of MgO
Mole of MgO = weight in gram / molecular mass
Weight in gram = 1.56 × 40.3045 = 63 g of Mg
I have rounded off the figures.