5 Simple Steps to Solve Solution Stoichiometry Problems

1. Figure out if it’s an M = n/V problem or a McVc = MdVd problem

Ernest Wolfe

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countdown.education

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2 min read

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Feb 12, 2017

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• M = n/V
• What the formula means:
• Molarity = number of moles / Volume (L)
• Key words:
• “react/reaction”
• “titrate/titration”
• What they’ll give you:
• They’ll give you the concentration/molarity, volume, or mass of one substance in a chemical reaction and you have to convert it to another substance
• two of the three variables between M, n, and V
• McVc = MdVd
• What the formula means:
• Molarity of Diluted solution x Volume of Diluted solution = Molarity of Concentrated solution x Volume of Concentrated solution
• this is a rearranged version of n=MV, basically saying moles of concentrated solute = moles of diluted solute
• Key words:
• “dilute”
• “add water” or “mix with water”
• “concentrated solution” or “stock solution”
• What they’ll give you:
• multiple molarities/concentrations for the same substance
• multiple volumes for the same substance
• a volume of water being added

2. See what you need to solve for

• “how many moles” / “what mass”
• solve for n → n = MV → convert moles to grams if necessary
• “what volume” / “how much solution”
• solve for V → V = n / M
• “what concentration” / “what molarity”
• solve for M → M = n / V
• “how would you prepare X solution”
• find out the number of moles of solute, then convert that to grams and say

3. For M=n/V, convert everything to moles

• Moles are the currency that allows you to convert from one substance to another
• Once you’ve converted to moles, you can use a mole bridge to switch between substances in a reaction based on their coefficients

4. For McVc=MdVd, solve for 3 of the 4 variables

• Sometimes, you will have to calculate the volume of the diluted solution
• Vd = Vc + Vwater

5. For “How would you prepare X solution” problems…

• If it’s an n=MV problem:
• Find the mass of solute after finding moles of solute and the volume of the desired solution
• Say “add enough water to X grams of [solute] to make Y liters of [desired solution]”
• If it’s a problem where you want to dilute a concentrated solution:
• Find the volume of concentrated solution and the volume of water added to make the diluted solution
• Say “add X liters of water to Y liters of [concentrated molarity]M solution”