See also some applications to equilibrium problems and the JavaScript Graphing Calculatorwhich extends the idea of the TI-85 to using data to solve for more than one unknown.
Entering the Equations
To get started, enter the following with upper/lower case exactly as shown below:
IDEAL = P * V = mol * Rg * T [ENTER] BOLT = nj/ni=e^(-dEij/(k*T)) [ENTER] FREE = dG = dHo - T(dSo - Rc ln(Q)) [ENTER]
This is a bit of a pain, as you have to use the [ALPHA] and [2nd][ALPHA] keys carefully. But the payoff will be worth the trouble, believe me! Note that we use 'mol' instead of 'n' in the ideal gas law because 'n' is not allowed as a variable on these calculators. We use 'Rg' instead of 'R' so that we can have one ideal gas constant (Rg) in units of L-atm/mol-K and one (Rc) in terms of Joules/mol-K.
Deleting Equations
The easiest way to delete an equation is to turn it into a simple variable by storing the value 0 in it. Here we do it for the unwanted equation 'JUNK':
0 [STO>] JUNK [ENTER]
0
'JUNK' will be no longer be an equation and will be gone from the solver equation menu.
Solving the Equations
Press
[2nd][SOLVER] to enter the solver [CLEAR] to clear out the current entry [2nd][RCL][IDEAL] to load the equation'IDEAL' should be one of the F-keys; press [MORE] if you don't see it.
eqn:P*V=mol*Rg*T P= V= mol= Rg= T=Some of these variables may have numbers after them, depending upon what already may be defined in your calculator. (The values will persist even after the calculator is turned off.) If the first line reads "exp:IDEAL" and the second line reads "exp=" that is OK. It just means you forgot to press [2nd][RCL]. You can use the equation, just not edit it. Just set exp=0 and continue.
Enter the values of all of the variables you know. Then go to the line for the variable you DON'T know and press [Clear] to erase whatever is there already and then [F5] to solve. We'll solve for volume (V) here:
eqn:P*V=mol*Rg*T eqn:P*V=mol*Rg*T
P=1 P=1
V= Press [F5]==> .V=22.40178759
mol=1 mol=1
Rg=.08205783 Rg=.08205783
T=273 T=273
bound={-1E99,1E99} bound={-1E99,1E99}
Note that you can do math as you define your variables! For example, say you were given that the temperature was 15 oC. You could add 273 'on the fly':
eqn:P*V=mol*Rg*T eqn:P*V=mol*Rg*T
P=1 P=1
V= Press [F5]==> .V=23.63265504
mol=1 mol=1
Rg=.08205783 Rg=.08205783
T=15+273 T=288
bound={-1E99,1E99} bound={-1E99,1E99}
Or maybe you want to solve for pressure, and the value you need is torr. For example,
What is the partial pressure of 1.0 mol of an ideal gas at 25oC if its volume is 2.0 L?
eqn:P*V=mol*Rg*T eqn:P*V=mol*Rg*T
P= Press [F5]==> .P=12.22661667
V=2 V=2
mol=1 mol=1
Rg=.08205783 Rg=.08205783
T=25+273 T=298
bound={-1E99,1E99} bound={-1E99,1E99}
This pressure is in atmospheres, so we need to multiply
eqn:P*V=mol*Rg*T eqn:P*V=mol*Rg*T
.P=12.22661667*760 Press [Enter]==> .P=9292.2286692
V=2 V=2
mol=1 mol=1
Rg=.08205783 Rg=.08205783
T=25+273 T=298
bound={-1E99,1E99} bound={-1E99,1E99}
Even better, you can use the conversions built into your calculator. We
could have done the following. (Note that "torr" are the same as "mmHg.")
To call up the conversions press [2nd][CONV][MORE][PRESS], then select the
current units of your value followed by the units you want to convert to.
eqn:P*V=mol*Rg*T eqn:P*V=mol*Rg*T
.P=12.22661667 atm>mmHg Press [Enter]==> .P=9292.2286692
V=2 V=2
mol=1 mol=1
Rg=.08205783 Rg=.08205783
T=25+273 T=298
bound={-1E99,1E99} bound={-1E99,1E99}
Note that once the conversions menu has been called up in this way, the solver
menu shifts up a notch. You can close the conversions menu by pressing [EXIT] or
you can leave it there an press [2nd][F5] now instead of [F5] to solve.
Editing Equations
If you find you need to edit an equation, here's how you can do it quickly. First set it up exactly the way you want by editing the first line of the solver. Test it out to make sure it does just what you want it to do. Exit the solver and press
[2nd][VARS][MORE][EQU]
VARIABLES:EQU FREE EQU >IDEAL EQU . . .Use the down-arrow key to find the equation you want to edit and press [ENTER]. Here we are editing the 'IDEAL' equation. You should see:
IDEAL(Alternatively, you could have just keyed this in yourself.) We're just going to define 'IDEAL' to be the value of the variable 'eqn', from the top line of the solver. Now press exactly this:
[ALPHA]=[2nd][RCL][2nd][ALPHA] e q n [ENTER][ENTER]
You should see the following on your screen:
IDEAL=P*V=mol*Rg*T
Done
Saving New Equations That Work
This same technique can be used for setting up equations in the first place. Just use the solver to get the equation the way you want it. Test it out and make sure it does what you want it to. Then exit the solver and key in the name you want to use for the equation--for example, 'IDEAL'. Then continue as above with
[ALPHA]=[2nd][RCL][2nd][ALPHA] e q n [ENTER][ENTER]
Using Variables Outside the Solver
The variables you define using the solver are still there after you exit the solver. Just use them like any other variable: 'x' is x, 'P' is P, 'dG' is dG, etc. For example, now I could key in (assuming I now have P in torr from my solver):
[LN] [ALPHA] P [ENTER]
ln P
9.13693370284
Constants
Note that several constants come with your calculator and can be used in any calculation or equation. Here are some of the more useful ones for chemistry:
| Avogadro Constant | Na | 6.0221367E+23 | 1/mol |
| Boltzmann Constant | k | 1.380658E-23 | J/K |
| Ideal Gas Constant | Rc | 8.31451 | J/mol/K |
| Plank's Constant | h | 6.6260755E-34 | J s |
| Speed of Light | c | 299792458 | m/s |
| Mass of the Electron | Me | 9.1093897E-31 | kg |
| Charge on the Electron | ec | 1.60217733E-19 | Coulombs |
Conversion Factors
Also, at least on the TI-85, under 'Conv' there are several useful conversions available:
| TEMP | oC, oK |
| PRESS | atm, mmHg(torr), inHg |
| ENERGY | J, cal, erg, eV |
You can combine the idea of a conversion and an equation. For example, from within the solver:
exp:x=(p-r) cal>J x= p= r=allows you to designate a product value (p) and a reactant value (r). Solving for 'x' gives you the difference in Joules if the values are in calories!
If you find other cool things about your calculator you think chemistry students would like to know about, let me know and I'll see if I can include them here.