Divide & Conquer- the dissection of a chemistry problem
Having trouble working out a problem in chemistry that deals with STP? Here is an example of how to go through a problem, step by step.
[Note: I do not 100% guarantee that any of my work is correct- it's just done to the best of my abilities- so please double check your answers if following my method to solve a similar problem]
*Remember significant figures! I'm not the best at sig. fig., but hopefully all my work is accurate.*
Reference to help you with mole calculations (you will need this later) - http://misterguch.brinkster.net/molecalculations.html
Problem: Calculate the volume of 1.25g of CH4 gas at STP.
How to solve the problem: First convert the number of grams of CH4 into moles, then use the Ideal Gas Law (pV = nRT) [P=pressure, V=volume, n=amount of substance in gas, R=gas constant, T=temperature]
Information you will need (you can figure all this out easily or know from memory) -
The molar mass of CH4 is 16.05 g/mol.
STP = 0 degrees C, or 273 degrees K and 1 atm (atm = unit of pressure).
Steps:
How many moles are in 1.25g of CH4 gas?
Make a T-chart with the given information (see reference site for help & explanations) -
(The following is supposed to be a T-chart, and the slanted lines are NOT division signs. They are part of the T-chart)
1.25 g of CH4 [given information] \ 1 mole CH4 [wanted units]
-----------------------------------------------------------------------------
[nothing goes here] \ [molar mass] 16.05 g of CH4
Units of grams cancel out. We are left with units of moles. Divide the top by the bottom, and give answer in moles.
= 7.78 x 10^-2 moles of CH4 (this is “n” in the ideal gas law equation)
Rearrange the ideal gas law equation to find “V”. V= nRT/ P
V = (7.78 x 10^-2 moles of CH4) [0.08206 (L)(atm)/ (mol)(K)] (273 degrees K) / (1 atm)
Because of the gas law constant, atm (unit of pressure), K (unit of temperature) and mol (numerical unit) cancel out, and we are left with units of L (unit of volume), which is what we want. Thus, our answer will be in units of L (liters).
Plug the problem into your scientific calculator.
I got 1.74 L.
[Note: I do not 100% guarantee that any of my work is correct- it's just done to the best of my abilities- so please double check your answers if following my method to solve a similar problem]
*Remember significant figures! I'm not the best at sig. fig., but hopefully all my work is accurate.*
Reference to help you with mole calculations (you will need this later) - http://misterguch.brinkster.net/molecalculations.html
Problem: Calculate the volume of 1.25g of CH4 gas at STP.
How to solve the problem: First convert the number of grams of CH4 into moles, then use the Ideal Gas Law (pV = nRT) [P=pressure, V=volume, n=amount of substance in gas, R=gas constant, T=temperature]
Information you will need (you can figure all this out easily or know from memory) -
The molar mass of CH4 is 16.05 g/mol.
STP = 0 degrees C, or 273 degrees K and 1 atm (atm = unit of pressure).
Steps:
How many moles are in 1.25g of CH4 gas?
Make a T-chart with the given information (see reference site for help & explanations) -
(The following is supposed to be a T-chart, and the slanted lines are NOT division signs. They are part of the T-chart)
1.25 g of CH4 [given information] \ 1 mole CH4 [wanted units]
-----------------------------------------------------------------------------
[nothing goes here] \ [molar mass] 16.05 g of CH4
Units of grams cancel out. We are left with units of moles. Divide the top by the bottom, and give answer in moles.
= 7.78 x 10^-2 moles of CH4 (this is “n” in the ideal gas law equation)
Rearrange the ideal gas law equation to find “V”. V= nRT/ P
V = (7.78 x 10^-2 moles of CH4) [0.08206 (L)(atm)/ (mol)(K)] (273 degrees K) / (1 atm)
Because of the gas law constant, atm (unit of pressure), K (unit of temperature) and mol (numerical unit) cancel out, and we are left with units of L (unit of volume), which is what we want. Thus, our answer will be in units of L (liters).
Plug the problem into your scientific calculator.
I got 1.74 L.