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To find the moles of Magnesium, dimensional was used to multiply the molar mass of Magnesium to its measured mass, the final result is 5.3475936 x 10-3 moles.

0.13g Mg×(1mol/(24.31g))=5.3475936×〖10〗^(-3)

After adding Hydrochloric acid and been heated on the hot plate, the total mass of evaporating dish plus Magnesium Chloride is 44.96 grams. By subtracting the mass of Magnesium ribbon 0.13 grams from the total mass 44.96 grams, the mass of Magnesium Chloride was found to be 0.56 grams.

44.96-44.40=0.56g

After knowing the mas of Magnesium chloride, by subtracting the mass of Magnesium ribbon from it, the mass of Chloride was found to be 0.43

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0.43g Cl×(1mol/(35.45g))=1.121297602×〖10〗^(-2) After knowing the moles of Magnesium and Chlorine, their mole ratio can be calculated by dividing the smaller moles, and then multiply a positive whole number to get the final empirical formula ratio of 9 Mg with 4 Cl.

(0.0121297602)/(0.0053475936)=2.26 (0.0053475936)/(0.0053475936)=1 Cl=2.26×4=9.07 Mg=1×4=4

As we know each mass of Magnesium and Chloride, the percent composition of Magnesium can be calculated when divide the mass of magnesium from the mass of Magnesium Chloride.

(0.13)/(0.56)=23.21428571%

After knowing the percent composition of Magnesium, and knowing the theoretical value as 25.5%, the percent error can be calculated by applying the formula.

(25.5-23.2142857)/(25.5)×100=8.963585451%