I've searched this forum front to back with no sign of an equation which helps in determining the right amount of reactants needed in a reaction.

Example: You want exactly 5 grams/kilograms of flashpowder, however you don't want to waste any of your valued magnesium in the process.

I took it upon myself to derive an equation which does so. I'll post the proof later if this is not deleted, or banned.

1) balance your equation.
2) plug and chug below: for a two reactant reaction, but can be expanded to fit any number of reactants

x = total composition needed
y = total amount of reactant 1 needed (general equation, just switch around the molar mass and coefficient)
a = molar mass of reactant 1
b = molar mass of reactant 2
c = molar coefficient of reactant 1 in balanced equation
d = molar coefficient of reactant 2 in balanced equation

y = x [ (ac) / sum of all (ac + bd) ]

Note: This can be used for any number of reactants; I chose two reactants for simplicity. For more reactants change the first 'ac' to fit nth reactant required, and sum up all of the reactants' (molar mass * molar coefficient).

Have fun saving time and materials :cool:

I think it'd be easier to just do the molar equations generally than use that.
Other than that I have a program that can do all that for me as well. All I do is put in reactants and products and desired weights.

Did not know there was a program which did that. But you fail to realize what this equation does, its completely different from "given so much carbon, how much KNO3 and sulfur is needed?"

Instead its "I want a combined weight of 'so and so' grams, how much of each (carbon, KNO3 and sulfur) do I need?"

But if you have a program which does so, please attach a copy, it would save more time when people start messing around with 5 to 7 reactants at once.

I have always just used molar masses, but if you could create a program that took into account for example the experimental yields compared to the theoretical including regards to density, phase changes and vapor pressure and how they affect the chemical equilibrium it could save a lot of time and effort. I have notebooks completely filled with chemical equations that could be eliminated by this sort of app.

FlamingPope, I might be able t write such a thing but you wouldn't like it. I think if you tinkered enough you could get a webby form (using javascript) that would do it for generic inorganic stuff.