Library Guides: Chemistry Textbook: Dimensional Analysis

Dimensional Analysis

By the end of this section, you will be able to:

Explain the dimensional analysis (factor label) approach to mathematical calculations involving quantities
Use dimensional analysis to carry out unit conversions for a given property and computations involving two or more properties

It is often the case that a quantity of interest may not be easy (or even possible) to measure directly but instead must be calculated from other directly measured properties and appropriate mathematical relationships. For example, consider measuring the average speed of an athlete running sprints. This is typically accomplished by measuring the time required for the athlete to run from the starting line to the finish line, and the distance between these two lines, and then computing speed from the equation that relates these three properties:

speed = \frac{distance}{time}

An Olympic-quality sprinter can run 100 m in approximately 10 s, corresponding to an average speed of

\frac{100 m}{10 s} = 10 m/s

Note that this simple arithmetic involves dividing the numbers of each measured quantity to yield the number of the computed quantity (100/10 = 10) and likewise dividing the units of each measured quantity to yield the unit of the computed quantity (m/s = m/s). Now, consider using this same relation to predict the time required for a person running at this speed to travel a distance of 25 m. The same relation among the three properties is used, but in this case, the two quantities provided are a speed (10 m/s) and a distance (25 m). To yield the sought property, time, the equation must be rearranged appropriately:

time = \frac{distance}{speed}

The time can then be computed as:

\frac{25 m}{10 m/s} = 2.5 s

Again, arithmetic on the numbers (25/10 = 2.5) was accompanied by the same arithmetic on the units (m/m/s = s) to yield the number and unit of the result, 2.5 s. Note that, just as for numbers, when a unit is divided by an identical unit (in this case, m/m), the result is “1”—or, as commonly phrased, the units “cancel.”

These calculations are examples of a versatile mathematical approach known as dimensional analysis (or the factor-label method). Dimensional analysis is based on this premise: the units of quantities must be subjected to the same mathematical operations as their associated numbers. This method can be applied to computations ranging from simple unit conversions to more complex, multi-step calculations involving several different quantities.

Conversion Factors and Dimensional Analysis

A ratio of two equivalent quantities expressed with different measurement units can be used as a unit conversion factor. For example, the lengths of 2.54 cm and 1 in. are equivalent (by definition), and so a unit conversion factor may be derived from the ratio,

\frac{2.54 cm}{1 in.} (2.54 cm = 1 in.) or 2.54 \frac{cm}{in.}

Several other commonly used conversion factors are given in Table 1.6.

Common Conversion Factors
Length	Volume	Mass
1 m = 1.0936 yd	1 L = 1.0567 qt	1 kg = 2.2046 lb
1 in. = 2.54 cm (exact)	1 qt = 0.94635 L	1 lb = 453.59 g
1 km = 0.62137 mi	1 ft³ = 28.317 L	1 (avoirdupois) oz = 28.349 g
1 mi = 1609.3 m	1 tbsp = 14.787 mL	1 (troy) oz = 31.103 g

Table 1.6

When a quantity (such as distance in inches) is multiplied by an appropriate unit conversion factor, the quantity is converted to an equivalent value with different units (such as distance in centimeters). For example, a basketball player’s vertical jump of 34 inches can be converted to centimeters by:

34 in. \times \frac{2.54 cm}{1 in.} = 86 cm

Since this simple arithmetic involves quantities, the premise of dimensional analysis requires that we multiply both numbers and units. The numbers of these two quantities are multiplied to yield the number of the product quantity, 86, whereas the units are multiplied to yield $\frac{in. \times cm}{in.}$ . Just as for numbers, a ratio of identical units is also numerically equal to one, $\frac{in.}{in.} = 1,$ and the unit product thus simplifies to cm. (When identical units divide to yield a factor of 1, they are said to “cancel.”) Dimensional analysis may be used to confirm the proper application of unit conversion factors as demonstrated in the following example.

EXAMPLE 1.8

Using a Unit Conversion Factor The mass of a competition frisbee is 125 g. Convert its mass to ounces using the unit conversion factor derived from the relationship 1 oz = 28.349 g (Table 1.6).

Solution Given the conversion factor, the mass in ounces may be derived using an equation similar to the one used for converting length from inches to centimeters.

x oz = 125 g \times unit conversion factor

The unit conversion factor may be represented as:

\frac{1 oz}{28.349 g} and \frac{28.349 g}{1 oz}

The correct unit conversion factor is the ratio that cancels the units of grams and leaves ounces.

\begin{matrix} x oz & = & 125 g \times \frac{1 oz}{28.349 g} \\ = & (\frac{125}{28.349}) oz \\ = & 4.41 oz (three significant figures) \end{matrix}

Check Your Learning Convert a volume of 9.345 qt to liters.

Answer:

8.844 L

Beyond simple unit conversions, the factor-label method can be used to solve more complex problems involving computations. Regardless of the details, the basic approach is the same—all the factors involved in the calculation must be appropriately oriented to ensure that their labels (units) will appropriately cancel and/or combine to yield the desired unit in the result. As your study of chemistry continues, you will encounter many opportunities to apply this approach.

EXAMPLE 1.9

Computing Quantities from Measurement Results and Known Mathematical Relations What is the density of common antifreeze in units of g/mL? A 4.00-qt sample of the antifreeze weighs 9.26 lb.

Solution Since $density = \frac{mass}{volume}$ , we need to divide the mass in grams by the volume in milliliters. In general: the number of units of B = the number of units of A $\times$ unit conversion factor. The necessary conversion factors are given in Table 1.6: 1 lb = 453.59 g; 1 L = 1.0567 qt; 1 L = 1,000 mL. Mass may be converted from pounds to grams as follows:

9.26 lb \times \frac{453.59 g}{1 lb} = 4.20 \times 10^{3} g

Volume may be converted from quarts to millimeters via two steps:

Convert quarts to liters.
$4.00 qt \times \frac{1 L}{1.0567 qt} = 3.78 L$
Convert liters to milliliters.
$3.78 L \times \frac{1000 mL}{1 L} = 3.78 \times 10^{3} mL$

Then,

density = \frac{4.20 \times 10^{3} g}{3.78 \times 10^{3} mL} = 1.11 g/mL

Alternatively, the calculation could be set up in a way that uses three unit conversion factors sequentially as follows:

\frac{9.26 lb}{4.00 qt} \times \frac{453.59 g}{1 lb} \times \frac{1.0567 qt}{1 L} \times \frac{1 L}{1000 mL} = 1.11 g/mL

Check Your Learning What is the volume in liters of 1.000 oz, given that 1 L = 1.0567 qt and 1 qt = 32 oz (exactly)?

Answer:

2.956 × 10⁻² L

EXAMPLE 1.10

Computing Quantities from Measurement Results and Known Mathematical Relations While being driven from Philadelphia to Atlanta, a distance of about 1250 km, a 2014 Lamborghini Aventador Roadster uses 213 L gasoline.

(a) What (average) fuel economy, in miles per gallon, did the Roadster get during this trip?

(b) If gasoline costs $3.80 per gallon, what was the fuel cost for this trip?

Solution (a) First convert distance from kilometers to miles:

1250 km \times \frac{0.62137 mi}{1 km} = 777 mi

and then convert volume from liters to gallons:

213 L \times \frac{1.0567 qt}{1 L} \times \frac{1 gal}{4 qt} = 56.3 gal

Finally,

(average) mileage = \frac{777 mi}{56.3 gal} = 13.8 miles/gallon = 13.8 mpg

Alternatively, the calculation could be set up in a way that uses all the conversion factors sequentially, as follows:

\frac{1250 km}{213 L} \times \frac{0.62137 mi}{1 km} \times \frac{1 L}{1.0567 qt} \times \frac{4 qt}{1 gal} = 13.8 mpg

(b) Using the previously calculated volume in gallons, we find:

56.3 gal \times \frac{$3.80}{1 gal} = $214

Check Your Learning A Toyota Prius Hybrid uses 59.7 L gasoline to drive from San Francisco to Seattle, a distance of 1300 km (two significant digits).

(a) What (average) fuel economy, in miles per gallon, did the Prius get during this trip?

(b) If gasoline costs $3.90 per gallon, what was the fuel cost for this trip?

Answer:

(a) 51 mpg; (b) $62

Key Concepts and Summary

Measurements are made using a variety of units. It is often useful or necessary to convert a measured quantity from one unit into another. These conversions are accomplished using unit conversion factors, which are derived by simple applications of a mathematical approach called the factor-label method or dimensional analysis. This strategy is also employed to calculate sought quantities using measured quantities and appropriate mathematical relations.