Question 1

During which type of radiation is a Helium nucleus emitted?

Accepted Answer

Alpha radiation is a form of radioactive decay that emits a helium nucleus (two protons and two neutrons). The original atomic nucleus has its mass number reduced by 4 during this type of radioactive decay.

Question 2

Which of the following is a property of bases?

Accepted Answer

Litmus paper is used to quickly and qualitatively determine whether a solution is acidic or basic. Red litmus paper turns blue when exposed to a basic solution. If it comes into contact with an acidic solution, it will remain red.

Question 3

F-21 has a half-life of 5 seconds. How long will it take a 50 g sample of F-21 to decay to a mass of 12.5 g?

Accepted Answer

The number of half-lives needed for 50 g to decay to 12.5 g is 2.
50 → 25 → 12.5
Therefore, you just multiply 5 seconds by 2 to get 10s as the amount of time it took for this mass to decay.

Question 4

Which of the following does pH measure?

Accepted Answer

This is a tricky question. pH is defined mathematically as -log[H⁺]. As you can see, it is a measure of the concentration of hydrogen ions. Although we use pH to determine the acidity of a solution, it does not give any indication as to the strength of an acid (that is what pk_a does).

Question 5

How many days will it take 128 kilograms of a radioactive substance to decay to 16 kilograms if it has a half-life of 32 days?

Accepted Answer

For a radioactive substance to decay from 128 kilograms to 16 kilograms, it must undergo 3 half-lives (128 →  64 → 32 → 16).
When the half-life of a radioactive substance is 32 days, then 3 half-lives would equal 96 days (3 × 32 = 96).

Question 6

The half-life of As-81 is 33 seconds. If only 12.5 g of a 100.0 g sample of As-81 is left, how long did it take for the original sample to decay to its current mass?

Accepted Answer

We first need to determine how many half-lives have occurred to create a mass of 12.5 g from the original 100 g sample.
100 → 50 → 25 → 12.5
This means that three half-lives have occurred. Since each half-life is 33 seconds, multiply 33 by 3 to obtain 99 seconds as the time it has taken for the As-81 to decay.

Question 7

What two functional groups do amino acids contain?

Accepted Answer

Amino acids are used during the synthesis of proteins by the body. There are 20 that are necessary to support life. The chemical structure of amino acids is composed of an amine and a carboxylic acid, hence the name "amino acid."

Question 8

If a fat has more than one double bond, what is it called?

Accepted Answer

'Unsaturation' refers to double bonds in the hydrocarbon chain of the fat. When only one double bond is present, the fat is called 'monounsaturated.' When more than one double bond is present, the fat is called 'polyunsaturated.' When no double bonds are present, the fat is called 'saturated.'

Question 9

Which of the following solvents are lipids insoluble in?

Accepted Answer

A lipid is defined as a molecule that is insoluble in water, but that is soluble in organic solvents such as ethanol, chloroform, and dichloromethane. Because of this, lipids are considered hydrophobic.

Question 10

Which of the following is emitted during beta decay?

Accepted Answer

Beta particles are high-energy, high-speed electrons that are emitted from an atomic nucleus. Beta decay can also emit a positron, and this is called β⁺ decay.

Question 11

Where are triglycerides stored?

Accepted Answer

When the body receives more calories than it needs, it stores them in the form of triglycerides in fat cells. The body releases the triglycerides in between meals when the body needs energy.

Question 12

Which of the following represents a normal blood pH level?

Accepted Answer

A person's pH should fall within the range of 7.35 to 7.45. Numerous health issues could arise if the pH deviates from this narrow range, including tremors, fainting, paralysis, or even death.
7.0 is considered neutral pH, in general, but is not the pH for human blood.
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Question 13

What state of matter are unsaturated fats at room temperature?

Accepted Answer

Unsaturated fats have alkenes in their fatty acid chain. The double bonds introduce bends in the long chains, which prevents the chains from packing efficiently. As a result, their melting points are much lower than saturated fats and are liquids at room temperature.

Question 14

Cesium-137 has a half-life of 30 years. If a 2.0 g sample of cesium-137 decays over 120 years, how much would remain?

Accepted Answer

Since cesium-137 has a half-life of 30 years, this means that half of the sample will disappear every 30 years.
Over a period of 120 years, cesium-137 will undergo 4 half-lives (120/30 = 4).
So, to get the final answer, we divide 2.0g in half four times:
30 years: 2 ÷ 2 = 1
60 years: 1 ÷ 2 = 0.5
90 years: 0.5 ÷ 2 = 0.25
120 years: 0.25 ÷ 2 = 0.125
So after 120 years, only 0.125 g would remain.

Question 15

Lemon juice contains citric acid. Which of the following pH values would lemon juice most likely have?

Accepted Answer

Since lemon juice contains an acid, it will most likely have an acidic pH. The only acidic pH in the options above is 2. In fact, the pH of citric acid is 2.2.
Below is a PH scale with the respective PH of common household items for your reference.
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Question 16

What are the two types of particles that can be emitted during beta decay?

Accepted Answer

Beta decay can occur in either β⁺, which produces a positronor β^-, which produces an electron. A positron is the antimatter counterpart to an electron and has a +1 charge.

Question 17

Which of the following pH values is acidic?

Accepted Answer

The pH scale is centered around a pH of 7, which is considered to be neutral. Values below 7 are considered to be acidic, while values greater than 7 are basic.
Since 5 is less than 7, it is an acidic pH value.
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Question 18

What percentage of an original sample of iodine-131 will remain after 64 days if iodine-131 has a half-life of 8 days?

Accepted Answer

First, determine the number of half-lives that will occur over a 64-day period. Do this by dividing the 64-day period by the 8-day half-life.
64 ÷ 8 = 8
This tells us that there will be 8 half-lives during this 64 day period. The half-life of a substance is the time is takes for that substance to decay. After each half-life, half of the starting amount will have decayed. In this case, every 8 days, half of the amount of iodine-131 decays.
Since this problem doesn't give us a starting amount and instead, asks us to find the percentage, we can start by plugging in 100g as the starting amount. This way, the ending amount will automatically give us the percentage that's left, because it is already out of 100.
For each half-life, we divide the starting amount in half, like this:
Half-life: 1
100 → 50
Half-life: 2
50 → 25
Half-life: 3
25 → 12.5
Half-life: 4
12.5 → 6.25
Half-life: 5
6.25 → 3.125
Half-life: 6
3.125 → 1.5625
Half-life: 7
1.5625 → 0.78125
Half-life: 8
0.78125 → 0.390625
The ending amount is 0.39g out of 100g that we started with. Therefore, the remaining amount after 8 half-lives (64 days) is 0.39% of the starting amount.

Question 19

Which of the following is not a lipid?

Accepted Answer

A lipid is a hydrophobic substance that is insoluble in water. All the above choices are lipids because they are insoluble in water, except for glucose, which readily dissolves in water.

Question 20

What type of radiation produces a particle with no mass?

Accepted Answer

Gamma radiation only produces a photon and energy during its decay. Since it has no mass, gamma radiation can travel much farther than alpha and beta radiation. Alpha radiation produces a helium nucleus and beta radiation produces an electron or a positron.

Exam 1: HESI-A2-Chemistry