Calculate the mass of KHP needed to react completely with 25 mL of a 0.10 M NaOH solution. Consider the reaction equation to be as shown below.

Question

 NOTE: The “P” in KHP ( 204 g/mol ) is an abbreviation for “phthalate”. It is not the element phosphorus.

Calculate the mass of KHP needed to react completely with 25 mL of a 0.10 M NaOH solution. Consider the reaction equation to be as shown below.  NOTE: The “P” in KHP ( 204 g/mol ) is an abbreviation for “phthalate”.  It is not the element phosphorus.

Answer 1

Hi Samuel,

There are 2 parts to solving this question:

1) Write out the balanced chemical equation so we know what ratios the elements are used in

2) Determine the number of moles of KHP needed, then convert that number from moles to grams.

 

1)Write a balanced chemical equation for this reaction.

This is an acid-base reaction where KPH is the acid and NaOH is the base.

We can write the left hand side of the equation as KPH + NaOH ↔

Because KPH is an acid, it will dissociate into KP^- and H⁺ in solution.  As a base, NaOH will dissociate into Na⁺ and OH^-.

The negative ion produced from the dissociation of KPH (KP^-) will bond with the positive ion produced by the dissociation of NaOH (Na⁺) to form NaKP, while the H⁺ from the acid will bond with the OH^- from the base to form H₂O.  This gives us NaKP and H₂O as the products of the reaction.

 

So the equation we get is:

KPH + NaOH ↔ NaKP + H₂O

Now we just need to check that the equation balances on both sides:

K - 1 in reactants in KPH and 1 in products in NaKP = balanced

P - 1 in reactants in KPH and 1 in products in NaKP = balanced

H - 1 in reactants in KPH and 1 in products in H₂O = balanced

Na - 1 in reactants in NaOH and 1 in products in NaKP = balanced

OH - 1 in reactants in NaOH and 1 in products in H₂O = balanced

All of the reactants and products balance, so we don't need to adjust the reaction coefficients.

Based on this equation, we need one mole of KPH to react with each mole of NaOH.

 

2) Determine the number of moles of KPH needed, and convert to grams

First, we need to know the number of moles of NaOH we have.  We have 25 mL of a 0.10 M solution of NaOH.  M= moles/liter, so we have 25 mL of a 0.10 mole/liter solution.

If we convert mL to liters, we get 25 mL x 1 L/1000 mL = 0.025 L

We multiply the number of liters by the moles/liter to get the number of moles:

0.025 L x 0.10 moles/L - 0.0025 moles NaOH

 

Since we know from our reaction equation above that we need 1 mole of KPH for each mole of NaOH, we know we need 0.0025 moles KPH.

 

Now we need to convert moles of KPH to grams KPH.  The problem states that the molecular mass of KPH is 204 g/mol.  

To get the number of grams, we need to multiply the number of moles by the molecular mass:

0.0025 moles KPH * 204g/mol = 0.51 g KPH.