# some help with chemistry word problem.?

What is the easiest way to look at a word problem and know what formula should be used? The temp is 90.5F pressure is 30.5 inches of Hg filled with 3.2 L of He. What volume will the balloon be when the temperature is -40.0F and the pressure is .25 atm?

### 2 Answers

- pisgahchemistLv 75 months ago
Gas laws....

If the question deals with the mass or moles of a substance at a specific temperature and pressure, then use the ideal gas equation:

PV = nRT

A handy variation of the ideal gas equation is PV = mRT / M (where m is mass, and M is molar mass).

If you are dealing with changes in pressure, volume or temperature for a confined gas, then use the combined gas law:

P1V1 / T1 = P2V2 / T2

If T is constant, then it reverts to Boyle's law: P1V1 = P2V2

If P is constant, then it reverts to Charles's law: V1/T1 = V2/T2

If V is constant, then it reverts to Amontons's law: P1/T1 = P2/T2

Note: there is no such gas law as "Gay-Lussac's law" expressed as P1/T1 = P2/T2. That is a myth which has been propagated for years by teachers and even textbook authors, people who should know better. Gay-Lussac investigated the same phenomena as did J. Charles. But Charles didn't publish his work. Gay-Lussac did and gave credit to Charles. In some places it is called the "law of Charles and Gay-Lussac."

Now, back to your question. You're dealing with a confined gas which is changing temperature, pressure and volume. Use the combined gas law.

P1V1/T1 = P2V2/T2

V2 = P1V1T2 / (T1P2)

V2 = 1.01 atm x 3.2L x 233K / 306K / 0.25atm

V2 = 9.8L ................... the answer can only be expressed to two significant digits

A comment about question construction. There was absolutely no need to muddy the water using Fahrenheit temperature and inches of mercury if the writer was asking about the combined gas law. If not careful, it becomes a problem about temperature and pressure conversion and not gases. A rule of thumb which question writers should follow is to test one concept at a time.

- Roger the MoleLv 75 months ago
"What is the easiest way to look at a word problem and know what formula should be used?"

In general, the only way I know what formula to use is experience.

In this case the Combined Gas Law works. (Although the full Ideal Gas Law could be used if you don't remember the Combined Gas Law.)

Remember when working with gases the temperature must always be in kelvin:

90.5F = 305.65 K

-40.0F = 233.15 K

(3.2 L) x (233.15 K / 305.65 K) x (30.5 inches Hg / (29.9213 inches Hg/atm)) / (0.25 atm) = 9.9504 L =

10 L