Question

multiple choice - counting atoms & elements

1. water is a substance that is essential for life and is made up of two hydrogen atoms and one oxygen atom per molecule. based on this chemical formula, h₂o, how many hydrogen atoms and how many oxygen atoms are present in a single water molecule?

a. 1 hydrogen, 2 oxygen
b. 2 hydrogen, 1 oxygen
c. 2 hydrogen, 2 oxygen
d. 1 hydrogen, 1 oxygen

1. table salt, which is often used to season food, has the chemical formula nacl. which elements can you find in table salt, based on this formula?

a. sodium and chlorine
b. nitrogen and carbon
c. sodium and calcium
d. nickel and chlorine

1. magnesium hydroxide is a compound sometimes used in antacids and has the chemical formula mg(oh)₂. how many oxygen atoms are contained in a single molecule of magnesium hydroxide?

a. 1
b. 2
c. 3
d. 4

1. glucose is a sugar molecule that provides energy to living organisms and has the chemical formula c₆h₁₂o₆. how many carbon atoms are in one molecule of glucose?

a. 6
b. 12
c. 18
d. 24

1. which of the following represents a compound, meaning a substance made of two or more different elements chemically combined in a fixed ratio?

a. o₂
b. nacl
c. h₂
d. he

1. carbon dioxide is a gas that humans exhale and plants use for photosynthesis. it has the chemical formula co₂. how many total atoms are in one molecule of carbon dioxide?

a. 1
b. 2
c. 3
d. 4

Explanation:

Step1: Analyze water formula

The formula for water is $H_2O$. The sub - script 2 on $H$ indicates 2 hydrogen atoms, and the lack of a sub - script on $O$ means 1 oxygen atom. So for question 1, the answer is 2 hydrogen, 1 oxygen.

Step2: Analyze table salt formula

For table salt with formula $NaCl$, $Na$ is sodium and $Cl$ is chlorine. So for question 2, the elements are sodium and chlorine.

Step3: Analyze magnesium hydroxide formula

In $Mg(OH)_2$, the sub - script 2 outside the parentheses applies to both $O$ and $H$. So there are 2 oxygen atoms in one molecule of magnesium hydroxide.

Step4: Analyze glucose formula

In $C_6H_{12}O_6$, the sub - script 6 on $C$ means there are 6 carbon atoms in one molecule of glucose.

Step5: Identify compound

A compound is made of two or more different elements chemically combined in a fixed ratio. $O_2$ is oxygen gas (diatomic element), $H_2$ is hydrogen gas (diatomic element), $He$ is helium (a noble gas, single - element). $NaCl$ is made of sodium and chlorine, so it is a compound.

Step6: Analyze carbon dioxide formula

In $CO_2$, there is 1 carbon atom and 2 oxygen atoms. So the total number of atoms in one molecule of carbon dioxide is $1 + 2=3$.

Answer:

1. B. 2 hydrogen, 1 oxygen
2. A. Sodium and Chlorine
3. B. 2
4. A. 6
5. B. $NaCl$
6. C. 3