Answer:
3.98 g
Explanation:
Step 1. Write the balanced chemical reaction. In this case, magnesium reacts with oxygen to produce magnesium oxide:

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Step 2. Calculate the number of moles of magnesium:

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Step 3. Calculate the number of moles of oxygen:

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Step 4. Identify the limiting reactant comparing the equivalents. Equivalent of Mg:

Equivalent of oxygen:

Therefore, Mg is the limiting reactant.
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Step 5. According to the stoichiometry of this reaction:

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Step 6. Convert the number of moles of MgO into mass:

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This is the balanced eq
N2 + 3H2 -> 2NH3
first you need to find mole of N2 by using
mol = mass ÷ molar mass.
mol N2= 20g ÷ (14.01×2)g/mol
=0.7138mol
then look at the coefficient between H2 and NH3.
it is N2:NH3
1:2
0.7138:0.7138×2
0.7138:1.4276 moles
moles of NH3 = 1.4276 moles
1 mole=6.02 x 10^23 atoms so how many moles are there in 3.0 x 10^23 we will cross multiply, 1 x 3.0 x 10^23 / 6.02 x 10 ^23. Which will give us 0.498 moles.
Hope this helped