High School Chemistry : Units

Example Questions

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Example Question #1 : High School Chemistry

What unit will be used to denote temperature in thermochemistry?

Celsius or Kelvin

Celsius

Kelvin

Joules

Fahrenheit

Kelvin

Explanation:

Temperature is the measure of the average kinetic energy of a system. Kelvin is the SI unit for temperature, and must be used because it is the only scale that does not have any negative numbers possible. At zero Kelvin (absolute zero) and there is absolutely no movement in the system, down to the atomic level.

While temperature is a means of measuring energy in a system, actual heat energy has the unit Joules.

Example Question #1 : High School Chemistry

Which of the following measurements contains 4 significant figures?

Explanation:

1. Any digit with a value 1-9 is always a significant figure

2. Any 0 between two digits 1-9 is always significant

3. If a number is greater than 1 and does not have a decimal point a 0 at the end is not significant because it is a tailing zero

4. If a number is less than 1 any 0 before a digit 1-9 is not significant because it is a leading zero

5. If there is a decimal point in any number, any 0 that follows a non-zero digit is always significant.

Thus,  has 4 significant figures. The first 0 is not significant because of rule 4. The digits 1, 9, and 6 are always significant (rule 1) and the last 0 is also significant because there is a decimal point and that 0 follows a non-zero digit (rule 5).

Example Question #2 : High School Chemistry

Which has the larger unit listed first?

Centigram and decigram

Kilojoule and megajoule

Picometer and millimeter

Decaliter and kiloliter

Microsecond and nanosecond

Microsecond and nanosecond

Explanation:

The following prefixes are ranked in order from largest to smallest with the power of 10 that the unit represents: peta (P)  > tera (T)  > giga (G)  > mega (M)  > kilo (k)  > hecta (H)  > deca (D)  > unit > deci (d)  > centi (c)  > milli (m)  > micro   > nano (n)  > pico (p)  > femto (f) . Thus a microsecond is one thousand times longer than a nanosecond.

Example Question #4 : Using Si Units

The measurement of  is equal to which of the following?

Explanation:

Units in the metric system can be converted using the correct factor of 10.

Example Question #1 : Using Si Units

A certain metal is weighed on a balance in a weighing boat. Before the metal is added, the mass of the weigh boat is recorded at . The final mass of the weigh boat and the metal is . The metal is added to a graduated cylinder filled with water to . After the metal is added to the water the graduated cylinder reads .

What is the density of the metal?

Explanation:

Density is equal to the mass of the object divided by the volume.

The mass of the metal is figured out by taking the mass of the metal and the weighboat  and subtracting the weighboat

The volume of the water is calculated by the displacement of the water so take the final volume and subtract the initial volume.

The density of the metal is calculated by taking the mass divided by the volume.

after rounding to 3 significant figures.

Example Question #1 : Units

An object is measured to be  tall. How tall is the object in inches?

Explanation:

You must use equalities and dimensional analysis to solve for the correct answer:

Each equality allows you to write a conversion factor 2 ways:

Example:  can be written as

and

To cancel out units if there is a unit on top it must be cancelled out by the same unit on the bottom.

To solve you must cancel out units until you get to the proper unit

Example Question #2 : High School Chemistry

Consider the following four samples:

of potassium

of lithium

of magnesium

of chlorine gas

Which of the given samples contains the most atoms?

They all have the same number of atoms

Chlorine

Lithium

Potassium

Lithium

Explanation:

It is important to note that the mass of a sample does not tell you the amount of atoms in the sample. The number of atoms in a sample is dependent on the number moles in a sample, given by Avogadro's number. Here is the number of moles for each sample:

Remember that chlorine is a diatomic mass, so each molecules contains two atoms. This doubles the molar mass for the conversion.

The sample with the greatest number of moles will also contain the most atoms. In this case, the sample of lithium results in the largest number of moles and, thus, the greatest number of atoms.

Example Question #1 : Measurements

Consider the reaction above. If you start with  of potassium bromide, how many moles of bromine are produced? How many molecules is this equal to?

Explanation:

In the chemical equation, the ratio of potassium bromide to bromine is 2:1, so for every 2 moles of , 1 mole of  is produced. Therefore, if we start with 4 moles of , we get 2 moles of . The number of molecules is equal to the number of moles times Avogadro's Number. Since we've determined the number of moles to be 2, the number of molecules is:

Example Question #2 : High School Chemistry

Convert the following amount from grams (g) to moles (m)

How many moles is  of ?

Explanation:

Use the periodic table to calculate the molecular weight of sodium hydroxide.

Next, use dimensional analysis to find the number of moles.

Example Question #1 : Measurements

How many moles of  are present in  of the substance?