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Phosphorus combines with oxygen to form an oxide that reacts with water to produce phosphoric acid,

Regents Chemistry Exam August 2022

Phosphorus combines with oxygen to form an oxide that reacts with water to produce phosphoric acid, which is an important industrial compound used to produce fertilizers. An unbalanced equation for the production of phosphoric acid is shown below. P4O10(s) + H2O(l) → H3PO4(aq) + energy

66 Balance the equation in your answer booklet for the production of phosphoric acid, using the smallest whole-number coefficients. [1]

Balancing the equation means the number of atoms of each elements must be equal on both sides.

We can only use coefficients (numbers in front of the formulas) for balancing. If you see the same element in multiple places on the same side, do it last (like oxygen appearing in two places on the left)

Let's start with phosphorus. There are 4 atoms on the left and 1 on the right, let's put 4 in front of H3PO4.

P4O10(s) + H2O(l) → 4H3PO4(aq)

Now, lets do hydrogen. There are 2 hydrogens on the left and 12 (4*3) on the right. We need to put 6 in in front of water

P4O10(s) + 6H2O(l) → 4H3PO4(aq)

Now lets check oxygen: 10+6 = 16 on the left and 4*4=16 on the right. Oxygen is balanced and our equation is balanced.

Answer: P4O10(s) + 6H2O(l) → 4H3PO4(aq)

67 Write the empirical formula of the solid reactant in the equation. [1]

The solid reactant is P4O10. Empirical formula is when atoms are in the smallest whole number ratio. Right now, the ratio is 4 to 10. Can we divide both numbers further? Yes, we can divide both number by 2. We will get 2 to 5, which can not be divided any further.

Answer: P2O5

68 Show a numerical setup for calculating the percent composition by mass of phosphorus in P4O10 (formula mass = 283.89 u). [1]

According to Table T, percent composition is

Percent Composition = (mass of part/ mass of whole)*100%

According to the Periodic Table, molar mass of phosphorus is 30.97376 u and there are 4 atoms of phosphorus in the formula. The mass of the whole is given to be 283.89 u.


69 Determine the oxidation state of phosphorus in the phosphoric acid. [1]

Phosphoric acid is H3PO4 as given by the equation. The oxidation number of H is +1 according to the Periodic Table and the oxidation number of O is -2 according to the Periodic Table. The sum of oxidation numbers in a compound must add up to zero if it is neutral.

We can make an equation where the oxidation number of phosphorus is x : +1(3)+x + (-2*4) = 0

3+x-8 = 0



Answer: +5


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