NY Regents Chemistry Exam Jan 2023

Boric acid, H3BO3, is heated to produce tetraboric acid, H2B4O7, and water. The equation below represents the reaction to form tetraboric acid.

The tetraboric acid is then used to make borax, which is used as a cleaning agent. Borax, H2B4O7 •10H2O, is a hydrate with a gram-formula mass of 381 grams per mole. A hydrate is a compound with water within its crystal structure. Borax has ten moles of water for every mole of Na2B4O7.

66. Explain why the formula for tetraboric acid is an empirical formula. [1]

Empirical formula is a formula where atoms of elements are in their simplest whole number ratios. In other words, we can not divide numbers any further in an empirical formula.

Answer: Na2B4O7 is an empirical formula because the atoms of the elements are in their simplest whole number ratios. 2,4, and 7 can not be divided any further.

67. Determine the number of moles of boric acid that react in the equation to produce 10 moles of water. [1]

The coefficient for boric acid is the equation is 4 and for water is 5. This means that for 4 moles of boric acid reacted, 5 moles of water will be produced.

If 10 moles of water are produced(twice as much), 8 moles of boric acid need to react.

Answer: 8 moles of boric acid.

68. Show a numerical setup for calculating the mass, in grams, of a 0.200-mole sample of borax. [1]

Table T on the Reference table gives mole calculations formula which is number of moles = given mass/gram formula mass. We are told in the given that gram-formula mass is 381 grams per mole.

Answer: 0.200moles = X/ (381g/mol)