As with all stoichometry problems, you must start by writing a balanced equation.
3 Zn (s) + Fe2O3 (s) → 2 Fe (s) + 3 ZnO (s)
Since you are given that there is 20.4g of Zinc metal as a reactant, that is your given. You will use this number to calculate your theoretical yield by using the mole ratio between Zinc and Iron.
20.4 g Zn • 1 mol Zn/65.38 g Zn • 2 mol Fe/3 mol Zn • 55.85g Fe/1 mol Fe = 11.6g Fe
Therefore, your theoretical yield is 11.6g.
Now, you divide your actual yield (10.8g) by your theoretical yield (11.6g) and multiply by 100 to get a percentage.
Explanation : Radiation is spontaneously emitted from nuclei of because this isotope of hydrogen is highly radioactive as compared to other isotopes of hydrogen namely; nuclei of and nuclei of .
They have much stable nucleus as compared to nuclei of .
The more it is unstable the more radiations will be emitted from its nucleus.