Chemistry and Measurements: Unit Conversions and Toxicology

Chemistry and Measurements

Introduction to Unit Conversions

In chemistry, accurate measurement and the ability to convert between different units are essential skills. Unit conversions allow scientists to communicate results and perform calculations using standardized systems.

• Unit conversion is the process of changing a measurement from one unit to another using conversion factors.

• Conversion factors are ratios that express how many of one unit are equal to another unit.

• Common systems include the metric system (SI units) and U.S. customary units.

Problem Solving Using Unit Conversion

Steps for Solving Unit Conversion Problems

Effective problem solving in chemistry involves a systematic approach to unit conversions:

1. Identify the given quantity and its units.

2. Determine the units needed in the answer.

3. Identify appropriate conversion factors that relate the given and needed units.

4. Set up the calculation so that units cancel appropriately, leaving only the desired unit.

Example: Converting Pounds to Kilograms

Suppose a person weighs 164 lb. What is the body mass in kilograms?

• Given: 164 lb

• Need: kilograms (kg)

• Conversion factor: $1\ \text{kg} = 2.20\ \text{lb}$

Calculation:

$164\ \text{lb} \times \frac{1\ \text{kg}}{2.20\ \text{lb}} = 74.5\ \text{kg}$

• Units of pounds cancel, leaving kilograms.

• Significant figures are maintained throughout the calculation.

Learning Check Example

• Question: A rattlesnake is 2.44 m long. How many centimeters long is the snake?

• Conversion factor: $1\ \text{m} = 100\ \text{cm}$

• Calculation: $2.44\ \text{m} \times \frac{100\ \text{cm}}{1\ \text{m}} = 244\ \text{cm}$

Using Two or More Conversion Factors

Multi-Step Unit Conversions

Some problems require the use of two or more conversion factors to reach the desired unit. Each factor is arranged so that units cancel sequentially until only the needed unit remains.

• Set up the problem so that each conversion factor cancels the previous unit.

• Continue until the final unit is obtained.

Example: Medication Dosage Calculation

A doctor's order prescribes a dosage of 0.150 mg of Synthroid. If tablets contain 75 mcg (micrograms) of Synthroid, how many tablets are required?

• Given: 0.150 mg Synthroid

• Need: number of tablets

• Conversion factors:

  • $1\ \text{mg} = 1000\ \text{mcg}$

  • $1\ \text{tablet} = 75\ \text{mcg}$

Calculation:

$0.150\ \text{mg} \times \frac{1000\ \text{mcg}}{1\ \text{mg}} \times \frac{1\ \text{tablet}}{75\ \text{mcg}} = 2\ \text{tablets}$

• First, convert milligrams to micrograms.

• Then, convert micrograms to number of tablets.

Learning Check Examples

• Question: How many minutes are in 1.4 days?

• Conversion factors:

  • $1\ \text{day} = 24\ \text{hours}$

  • $1\ \text{hour} = 60\ \text{minutes}$

• Calculation: $1.4\ \text{days} \times \frac{24\ \text{hours}}{1\ \text{day}} \times \frac{60\ \text{minutes}}{1\ \text{hour}} = 2016\ \text{minutes}$

• Question: If your pace on a treadmill is 65 meters per minute, how many minutes will it take to walk 4.0 kilometers?

• Conversion factors:

  • $1\ \text{km} = 1000\ \text{m}$

• Calculation: $4.0\ \text{km} \times \frac{1000\ \text{m}}{1\ \text{km}} = 4000\ \text{m}$; $\frac{4000\ \text{m}}{65\ \text{m/min}} \approx 61.5\ \text{min}$

Chemistry Link to Health: Toxicology

Understanding Toxicity and LD50

Toxicology is the study of the adverse effects of chemicals on living organisms. A fundamental concept is that "the dose makes the poison"—the amount of a substance determines its potential for harm.

• LD50 (Lethal Dose, 50%) is a standard measure of toxicity, representing the dose required to kill 50% of a test population (usually laboratory animals).

• LD50 values are typically expressed in milligrams or micrograms of substance per kilogram of body mass ($\text{mg/kg}$ or $\mu\text{g/kg}$).

• Lower LD50 values indicate higher toxicity.

Example: The LD50 of caffeine is 192 mg/kg, meaning a dose of 192 mg per kg of body weight is lethal to 50% of test animals.

Comparing Toxicity Using LD50 Values

LD50 values allow for the comparison of the relative toxicity of different substances. Substances with lower LD50 values are more toxic.

Substance	LD50 (mg/kg, rats)	Toxicity
Dioxin (pesticide)	3	Very high
Caffeine	192	Moderate
Additional info: Table may include other substances such as ethanol, aspirin, etc., with higher LD50 values indicating lower toxicity.	Additional info: ...	Additional info: ...

Risk assessment in toxicology involves exposing laboratory animals to substances and monitoring health effects to determine safe exposure levels for humans.