2 - Chemistry and Measurements

Cubic Centimeter

(abbreviated as cm^3 or cc) is the volume of a cube whose dimensions are 1 cm on each side. A cubic centimeter has the same volume as a milliliter, and the units are often used interchangeably.

Rules for Rounding Off

1. If the first digit to be dropped is 4 or less, then it and all following digits are simply dropped from the number. 2. If the first digit to be dropped is 5 or greater, then the last retained digit of the number is increased by 1.

Metric System

A system of measurement used by scientists and in most countries of the world.

Equality

An equality shows the relationship between two units that measure the same quantity of length, volume, mass, or time.

Hydrometer

An instrument called a hydrometer is often used to measure the specific gravity of fluids such as battery fluid or a sample of urine.

Measuring Length

An ophthalmologist may measure the diameter of the retina of an eye in centimeters (cm), whereas a surgeon may need to know the length of a nerve in millimeters (mm). When the prefix centi is used with the unit meter, it becomes centimeter, a length that is one-hundredth of a meter (0.01 m). When the prefix milli is used with the unit meter, it becomes millimeter, a length that is one-thousandth of a meter (0.001 m). There are 100 cm and 1000 mm in a meter.

Conversion Factor

Any equality can be written as fractions called conversion factors with one of the quantities in the numerator and the other quantity in the denominator. Be sure to include the units when you write the conversion factors. Two conversion factors are always possible from any equality. Two conversion factors can be written for any relationship in the metric or U.S. system. A percentage is written as a conversion factor by expressing matching units as the parts in 100 parts of the whole.

Mililiter (mL)

In a laboratory or a hospital, chemists work with metric units of volume that are smaller and more convenient, such as the milliliter (mL). There are 1000 mL in 1 L.

Significant Figures (SFs)

In a measured number, the significant figures (SFs) are all the digits including the estimated digit. Nonzero numbers are always counted as significant figures. However, a zero may or may not be a significant figure depending on its position in a number.

Addition and Subtraction with Measured Numbers

In addition or subtraction, the final answer is written so that it has the same number of decimal places as the measurement having the fewest decimal places.

Multiplication and Division with Measured Numbers

In multiplication or division, the final answer is written so that it has the same number of significant figures (SFs) as the measurement with the fewest SFs.

Kelvin (K) temperature scale

In the SI system, temperature is measured using the Kelvin (K) temperature scale on which the lowest possible temperature is 0 K. A unit on the Kelvin scale is called a kelvin (K) and is not written with a degree sign.

Celsius (°C) temperature scale

In the metric system, temperature is measured using Celsius temperature. On the Celsius (°C)temperature scale, water freezes at 0°C and boils at 100°C, while on the Fahrenheit (°F) scale, water freezes at 32°F and boils at 212°F.

Gram (g)

In the metric system, the unit for mass is the gram (g), which is used for smaller masses. There are 1000 g in 1 kg. It takes 2.20 lb to make 1 kg, and 454 g is equal to 1 lb.

Second (s)

Of these, the SI and metric unit of time is the second (s). The standard now used to determine a second is an atomic clock.

Kilogram (kg)

The SI unit of mass, the kilogram (kg), is used for larger masses such as body mass.

Density

The mass and volume of any object can be measured. If we compare the mass of the object to its volume, we obtain a relationship called density. Density = mass of substance/volume of substance The units of density can be used to write conversion factors that convert between the mass and volume of a substance.

Meter (m)

The metric and SI unit of length is the meter (m). A meter is 39.4 inches (in.), which makes it slightly longer than a yard (yd).

Prefix

The special feature of the SI as well as the metric system is that a prefix can be placed in front of any unit to increase or decrease its size by some factor of 10.

Density of A Solid

The volume of a solid can be determined by volume displacement. When a solid is completely submerged in water, it displaces a volume that is equal to the volume of the solid.

Measuring Volume

Volumes of 1 L or smaller are common in the health sciences. When a liter is divided into 10 equal portions, each portion is a deciliter (dL). There are 10 dL in 1 L. Laboratory results for blood work are often reported in mass per deciliter.

Time

We typically measure time in units such as years (yr), days, hours (h), minutes (min), or seconds (s).

Measuring Mass

When you go to the doctor for a physical examination, your mass is recorded in kilograms, whereas the results of your laboratory tests are reported in grams, milligrams (mg), or micrograms (μg or mcg). A kilogram is equal to 1000 g. One gram represents the same mass as 1000 mg, and one mg equals 1000 μg (or 1000 mcg).

Centimeter (cm)

a smaller unit of length, is commonly used in chemistry and is about equal to the width of your little finger. For comparison, there are 2.54 cm in 1 in.

Measured Numbers

are the numbers you obtain when you measure a quantity such as your height, weight, or temperature.

Exact Numbers

are those numbers obtained by counting items or using a definition that compares two units in the same measuring system. Exact numbers are not measured, do not have a limited number of significant figures, and do not affect the number of significant figures in a calculated answer.

Prefixes That Decrease the Size of the Unit

deci- (d) 10^-1 centi- (c) 10^-2 mili- (m) 10^-3 micro- (μ) 10^-6 nano- (n) 10^-9 pico- (p) 10^-12

Specific Gravity (sp gr)

is a relationship between the density of a substance and the density of water. Specific gravity is calculated by dividing the density of a sample by the density of water, which is 1.00g/mL at 4°C. A substance with a specific gravity of 1.00 has the same numerical value as the density of water (1.00g/mL). Specific gravity = density of sample/density of water Specific gravity is one of the few unitless values you will encounter in chemistry.

Liter (L)

is slightly larger than the quart (qt), (1L = 1.06qt).

Volume

is the amount of space a substance occupies.

Mass

of an object is a measure of the quantity of material it contains.

International System of Units

or Système International, is the official system of measurement throughout the world except for the United States.

Temperature

tells us how hot something is, how cold it is outside, or helps us determine if we have a fever.

Prefixes That Increase the Size of the Unit

tera- (T) 10^12 giga- (G) 10^9 mega- (M) 10^6 kilo- (k) 10^3