The equation of the reaction of the lab is:
2 Mg + O₂ -----> 2 MgO
We will use this equation to find the percent yield:
% yield = actual yield/theoretical yield * 100
We can calculate from the given data the actual yield of each trial.
Mass of MgO produced in trial 1 = 27.272 g - 26.690 g
Mass of MgO produced in trial 1 = 0.582 g
Actual yield 1 = 0.582 g
Mass of MgO produced in trial 2 = 27.351 g - 26.681 g
Mass of MgO produced in trial 2 =
Actual yield 2 = 0.67 g
Now we know the actual yield of both trials, but we still have to find the theoretical yield.
2 Mg + O₂ -----> 2 MgO
According to this equation, 2 moles of Mg (with excess O₂) will produce 2 moles of MgO, so their relationship is 1 to 1.
To find the theoretical yield we will use that relationship. First we have to find the number of moles of Mg that reacted in each trial.
mass of Mg in trial 1 = 27.044 g - 26.690 g
mass of Mg in trial 1 = 0.354 g
mass of Mg in trial 2 = 27.089 g - 26.681 g
mass of Mg in trial 2 = 0.408 g
molar mass of Mg = 24.31 g/mol
moles of Mg in trial 1 = 0.354 g/(24.31 g/mol)
moles of Mg in trial 1 = 0.014561 moles
moles of Mg in trial 2 = 0.408 g/(24.31 g/mol)
moles of Mg in trial 2 = 0.016783 moles
We said before that 1 mol of Mg will produce 2 moles of MgO. Let's find the number of moles that should have been produced in each trial.
moles of MgO in trial 1 = 0.014561 moles of Mg * 1 mol of MgO/(1 mol of Mg)
moles of MgO in trial 1 = 0.014561 moles
moles of MgO in trial 2 = 0.016783 moles of Mg * 1 mol of MgO/(1 mol of Mg)
moles of MgO in trial 2 = 0.016783 moles
Now we can find the theoretical yield of each trial using the molar mass of MgO:
molar mass of MgO = 24.31 g/mol + 16.00 g/mol
molar mass of MgO = 40.31 g/mol
mass of MgO theoretically produced in trial 1 = 0.014561 moles * 40.31 g/mol
mass of MgO theoretically produced in trial 1 = 0.5870 g
theoretical yield trial 1 = 0.5870 g
mass of MgO theoretically produced in trial 2 = 0.016783 moles * 40.31 g/mol
mass of MgO theoretically produced in trial 2 = 0.6765 g
theoretical yield trial 2 = 0.6765 g
Finally we can find the percent yield for each trial:
% yield trial 1 = actual yield trial 1/theoretical yield trial 1 * 100
% yield trial 1 = 0.582 g/0.5870 g * 100
% yield trial 1 = 99.15 %
% yield trial 2 = actual yield trial 2/theoretical yield trial 2 * 100
% yield trial 2 = 0.67 g/0.6765 g * 100
% yield trial 2 = 99.04 %
Answer: the % yield for trial 1 is 99.15 % and for trial 2 is 99.04 %