ap chemistry question

Test Preparation
1

<p>i have an ap chem test on friday, and have no clue how to do this problem:</p>

<p>thanks in advance!!!! :)</p>

<p>KHP (potassium hydrogen phthalate KHC8H4O4) is often used as a primary standard for sodium hydroxide standardization: </p>

<p>HC8H4O4-(aq) + OH-(aq) -> C8H4O42-(aq) + H2O(l) </p>

<p>If a 0.4856 gram sample of KHP is dissolved in sufficient water to prepare 250 mL of solution, and 25 mL of the solution requires 18.76mL of sodium hydroxide solution to reach the equivalence point, what is the molarity of the sodium hydroxide?</p>

<p>the numbers are subscripts.</p>

<p>thanks.</p>

2

<p>let x = .48__ g of KHP then 25ml/250ml = 1/10 of x. So let x/10 represent the amount of g of KHP is 25ml of solution. Then g/molar mass = moles. Now you know how many moles of KHP you have. Then Y moles of KHP * 1 mole OH/1 mole KHP (assuming 1 to 1 mole ratio, didnt look at reaction) = Y moles of OH which is in 18.75ml. Turn that into L and then molarity = moles/liter. So take Ymoles OH / (18.74 ml * 1L / 1000ml). I hope this helps, I didnt feel like doing the math out, but this is the process.</p>

3

<p>ok thanks.</p>

<p>damn, this test is going to suck.</p>

4

<p>so the molarity (M) is the same for OH and Na ions.... thus NaOH is the same molarity as the OH answer I get?</p>

<p>thanks.</p>

5

<p>yep... that makes sense.</p>

<p>thanks a bunch.</p>

6

<p>Simplified instructions:</p>

<p>First convert the grams of KHP to mols of KHP using the molecular mass, then use the BALANCED equation given, (1 mol OH- / 1 mol KHP) to get the mols of NaOH used (1 mol NaOH / 1 mol OH-) - and you know how many L of NaOH you added, so the molarity is simply mol NaOH / L NaOH... (mol/L)
Sometimes on these problems they'll try to trick you by giving you an unbalanced equation, so always check</p>

7

<p>i got it, bro.</p>

<p>thanks a ton.. both of you! :D</p>