Counting Atoms STAAR Bootcamp W3

Barium sulfate, BaSO₄, is a white crystalline solid that is insoluble in water. It is used by doctors to diagnose problems with the digestive system. Barium hydroxide, Ba(OH)₂, is also a white crystalline solid and is used in wastewater treatment.

How many more oxygen atoms are represented in the formula for barium sulfate than in the formula for barium hydroxide?

(a)  

Answer explanation

Barium sulfate (BaSO₄) has 4 oxygen atoms, while barium hydroxide (Ba(OH)₂) has 2 oxygen atoms. The difference is 4 - 2 = 2. However, the question asks how many more oxygen atoms are in barium sulfate than in barium hydroxide, which is 2.

Tags

NGSS.MS-PS1-1

Oxygen and oxygen-containing compounds are involved in many different reactions. Which balanced equation represents a reaction that involves 14 atoms of oxygen?

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Answer explanation

The total of oxygen atoms on one side of the chemical formula is 14. Coefficient multiples to all while subscript is only for that atom it is attached to. If there is no subscript, then there is one atom.

Tags

NGSS.MS-PS1-2

NGSS.MS-PS1-5

The names and chemical formulas of four substances are shown in the chart. Which substance listed in the chart is made up of the most atoms?

• Carbonic acid

• Nitric acid

• Phosphoric acid

• Sulfuric acid

Answer explanation

Phosphoric acid is made up of the most atoms among the substances listed in the chart with 8 atoms. Coefficient multiples to all while subscript is only for that atom it is attached to. If there is no subscript, then there is one atom.

Tags

NGSS.MS-PS1-1

A compound secreted by honeybees to paralyze intruders in their hive is known as 2-heptanone. The formula for this compound is shown.

How many atoms of each element are in one molecule of 2-heptanone?

• 4 carbon atoms, 11 hydrogen atoms, and 1 oxygen atom

• 7 carbon atoms, 14 hydrogen atoms, and 1 oxygen atom

• 7 carbon atoms, 11 hydrogen atoms, and 8 oxygen atoms

• 6 carbon atoms, 14 hydrogen atoms, and 3 oxygen atoms

Answer explanation

This is because the formula for 2-heptanone indicates that it contains 7 carbon atoms, 14 hydrogen atoms, and 1 oxygen atom. Coefficient multiples to all while subscript is only for that atom it is attached to. If there is no subscript, then there is one atom.

Tags

NGSS.MS-PS1-1

Which chemical reaction involves the fewest oxygen atoms?

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Answer explanation

There are 6 oxygen atoms on each side of the chemical formula. Coefficient multiples to all while subscript is only for that atom it is attached to. If there is no subscript, then there is one atom.

A student is using colored beads to make a model of aluminum sulfate, Al₂(SO₄)₃. Aluminum atoms are represented by blue beads, sulfur atoms by yellow beads, and oxygen atoms by green beads.

What combination of beads should the student use for the modes

• 6 blue, 3 yellow, and 7 green

• 2 blue, 1 yellow, and 4 green

• 2 blue, 3 yellow, and 12 green

• 6 blue, 12 yellow, and 12 green

Answer explanation

This combination represents the correct ratio of aluminum, sulfur, and oxygen atoms in the compound. Coefficient multiples to all while subscript is only for that atom it is attached to or all that is inside the paratheses. If there is no subscript, then there is one atom.

Tags

NGSS.MS-PS1-1

A chemical equation for the reaction between calcium hydroxide, Ca(OH)₂, and sulfuric acid, H₂SO₄, is shown. The masses of some of the reactants and products are given in grams.

How may grams of sulfuric acid were used in the reaction?

• 62 g

• 98 g

• 36 g

• 100 g

Answer explanation

The total mass remains constant because mass is conserved in a chemical reaction. So the total of each side should be the same.

Tags

NGSS.MS-PS1-2

NGSS.MS-PS1-5

Two solutions are mixed and react to produce the solid precipitate AgCl. The chemical reaction is shown by the balanced chemical equation.

What happens to the total mass as the reaction takes place?

• The mass increases.

• The mass decreases.

• The mass remains constant.

• The mass first decreases and then increases.

Answer explanation

The total mass remains constant because mass is conserved in a chemical reaction.

Tags

NGSS.MS-PS1-2

NGSS.MS-PS1-5

The chemical formula for sodium sulfate is Na₂SO₄. How many sulfur atoms are in the formula for sodium sulfate?

• 1

• 2

• 6

• 7

Answer explanation

If there is no subscript, then there is one atom. In this formula, there is only 1 sulfur atom.

Tags

NGSS.MS-PS1-1

Which of these formulas contain equal numbers of nitrogen atoms?

• Formulas I and III

• Formulas I and IV

• Formulas II and III

• Formulas I, II, and V

Answer explanation

Coefficient multiples to all while subscript is only for that atom it is attached to or all that is inside the paratheses. If there is no subscript, then there is one atom. DO NOT forget the separated same atoms need to be added together (nitrogen).

Tags

NGSS.MS-PS1-1