The half-life of a radioactive element is 1250 years. what percent of atoms remain after 7500 years?

8. 13.3g of Oxygen is reacted with 10.0g of Magnesium

9. 49.6g of Ammonia (NH₃) reacts with 106.5g of HCl

   The law of conservation of mass is observed in the reaction

Explanation:

8. The Relative Atomic Masses of the elements involved are:  

   Mg=24, O=16

   The equation for the reaction is given as  

   2Mg(s) + O₂(g) → 2MgO(s)  

   From the equation above

   Let's assume that the amount in gram of Oxygen required to react with 10.0g of Mg is x

   2(24)g of Magnesium is required to react with (16 x 2)g of Oxygen

   10.0g of Magnesium should react with x g of Oxygen  

   ∴ x = (10.0 x (16 x 2))/2(24) = (10.0 x 32)/48 = 13.3g of Oxygen

9.  The Relative Atomic Masses of the elements involved in the reaction are  

    H=1, Cl=35.5, N=14  

    The equation for the reaction is given as  

    HCl(g) + NH₃(g) → NH₄Cl(s)  

    From the equation above

    Let's say y equals the amount in grams of Ammonia involved in the reaction  

    (1 + 35.5)g of HCl reacts with (14+(1 x 3))g of Ammonia  

    106g of HCl should react with y g of Ammonia

∴ y = (106 x (14 + (1 x 3)))/(1+35.5) = (106 x 17)/36.5 = 49.6g of Ammonia

The  law of conservation of mass states that the total mass in a closed system (reaction) remains constant over time (i.e mass is neither gained or lost). In other words, the total mass of the reactant equals the total mass of the product.

Based on the reaction above, the total mass of the reactant (i.e HCl and NH₃) is 53.5g which is the same as the product (i.e NH₄Cl) which is also 53.5g.