Anonymous

# PLEASE! convert 1.00g/cm^3 to lb/ft^3?

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1.00 g / cm³ x ( 1 lb / 454 g) x ( 2.54 cm / in)³ x (12 in / ft)³ = 62.4 lb/ft³

Tiffany, I highly recommend you spend 10 minutes and read the rest of this answer. Unit conversions is important. This process will help you immensly....once you have completely read through this, look at how I converted again... 1 lb = 454 g.... so (1 lb / 454 g) = 1 ... (2.54 cm / 1 in) = 1... (12 in / ft) = 1...

so I essentially multiplied 1.00 g / cm³ x 1 x 1 x 1... right? and used forms of "1" that changed the units...

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The process I used is called "unit factor method" aka "factor label method" and sometimes called "dimensional analysis"... it is one of the most important techniques you will learn in chemistry. it allows you to do unit conversions while watching the units cancel. Which prevents many common errors. It also allows you to change compound units easily...like kg m/s² to lb ft / s² for example...

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"unit factor method" explained

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take each of theses rules 1 at a time and read them thoroughly until you understand them. once you do, move on to the next...email me if you have questions...(you can email me via the link on my profile page).

rule 1) units on top and bottom of a fraction cancel.

examples:

in / in = 1

ft / ft = 1

cm / cm cancels.

sec / sec cancel

sec² / sec² cancel

m³ / m³ cancels

got that down?

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rule 2) any number x 1 = that number

examples:

4 x 1 = 4

4 in x 1 = 4 in

5280 ft x 1 = 5280 ft

9.8 m / s² x 1 = 9.8 m / s²

easy right?

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rule 3) any equality can be rearranged to = 1

examples:

2.54 cm = 1 in

if we divide both sides by 2.54 we get

2.54 cm / 2.54 cm = 1 in / 2.54 cm

1 = 1 in / 2.54 cm

12 in = 1 ft

12 in / 12 in = 1 ft / 12 in

1 = 1 ft / 12 in

also... since 1/1 = 1....

1 / 1 = 1 / (1 ft / 12 in)

1 = 12 in / 1 ft

meaning for any equality of the form a = b, .....a / b = b / a = 1

a/b and b/a are called unit factors because they = unity and we will be using them in rules 4 and 5 to factor (or more precisely "change" units)

ok? this may be a bit more complicated than the first two rules but if you play around with it for a while I'm sure you will get it.

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rule 4) units can be changed by multiplying by an appropriate "unit factor".

this essentially combines rules 1, 2, and 3

example....

10 in = ? cm

10 in x 1 = 10 in right?

and 2.54 cm / 1 in = 1 right?

substituting

10 in x (2.54 cm / 1 in ) = 10 x 2.54 x in x cm / in = 25.4 cm

because inches cancel....

3 ft = ? in

3 ft x 1 = 3 ft x (12 in / ft) = 36 in

here is an important trick... if the units you want to cancel are on the top, put the matching units for the "unit factor" on the bottom.. and vice versa. if on the bottom, then put the matching ones one the top....

this will definitely take practice...

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rule 5) exponents....

since 1 to any power = 1... any "unit factor" raised to any power = 1

examples:

1² = 1

(2.54 cm / in) = 1

(2.54 cm / in)² = 1² = 1

similiarly

(2.54 cm / in)³ = 1³ = 1

how you use this is like this...

10 in³ = ? cm³

10 in³ x 1 = 10 in³ x (2.54 cm / in)³

= 10 in³ x (2.54 cm)³ / (1 in)³

= 10 in³ x 16.4 cm³ / 1 in³

= 164 cm³

645 ft³ = ??? in³

645 ft³ x (12 in / ft)³ = 1.11 x 10^6 in³

do you see that? 645 x 12³ = 1.11 x 10^6... ft³ / ft³ cancels... remaining units are in³

645 ft³ = ??? m³

645 ft³ x (12 in / ft)³ x (2.54 cm / in)³ x (1 m / 100 cm)³ = 18.3 m³

645 x 12³ x 2.54³ / 100³ = 18.3...all the units have cancelled except for m³ right?

and that's all there is to it...