Quick neutralization question

[discowisco]

How many ml of 0.4 M naoh are required to neutralize 100 ml 0.25 M h2s04?

So for neutralization problems I understand you need an Equivalent amount of moles of acid and base and so my question is for neutralization problems do you guys write out the chemical equation or is there an easier way to see that you need tojltiply by 2 equivalents for the naoh?

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[Cptn Loko]

Once you have done enough of this type of problem, then it becomes apparent that 2 equivalents of NaOH are required. The easiest way to see that is to realize that H2SO4 has 2 acidic protons, each requiring one equivalent of OH- (strong base) to neutralize. Thus, the basic idea is OH- + H+ --> H2O.

H3PO4 has 3 hydrogens, hence requiring 3 equivalents of OH- to neutralize.
And the simplest example, HCl has 1 proton, requires 1 equivalent of OH- to neutralize.

Writing out the equation would waste valuable testing time. The harder part of this type of problem once you get the concept tends to be not screwing up your math.

 

[Tarasaurusrex]

There are 2 ways to do this problem:

1. H2SO4 + 2NaOH --> Na2SO4 + 2H2O

Molarity = moles solute/L solution

0.100 L x 0.25 M H2SO4 = 0.025 mol H2SO4

0.025 mol H2SO4 x (2mol NaOH/1 mol H2SO4) = 0.050 mol NaOH

0.050 mol NaOH/0.4 M = 0.125 L x 1000 = 125 mL

or

2. Molar ratio of reactants is 2:1 so, use 2M1V1 = M2V2

2(0.100 L)(0.25 M H2SO4) = 1(0.4 M NaOH)V2

V2 = 0.125 L x 1000 = 125 mL NaOH

 

[brood910]

EDIT: NVM

 

[Cmdr_Shepard]

On the actual MCAT, you're not gonna wanna go through all that math. What you need to be comfortable with is estimating the math required for these types of questions.

For this one, I'd just say to myself that the 100 ml 0.25M acid is equal to 100 ml of 0.5M protons (2 protons each mole of acid). So you're gonna need at least 100 ml of 0.5M strong base. If you've got only 0.4M strong base, then you've got to increase your volume by some amount to compensate.

On the test, there will only be one answer close to, and greater than, 100 ml. Likely they'd give you 100 ml, 175 ml, 125 ml and 75 ml to choose from.

Edit: You must remember that a slow pace for each question can bring you down from 14 to 9.