Aim

To standardize a sodium hydroxide (NaOH) solution against a primary standard acid [Potassium Hydrogen Phthalate (KHP)] using phenolphthalein as an indicator.

Variables

Equipment

1. 2 g of KHP
2. Two 100 cm³ Beakers (One for making the KHP solution, one for pouring NaOH into the burette)
3. 1 Digital Balance (up to 2 decimal places accuracy)
4. 1 Stirring rod
5. 1 Funnel
6. 100 cm³ volumetric flask
7. 100 ml distilled water
8. Conical flask
9. Phenolphthalein
10. 100 cm³ burette
11. Sodium Hydroxide

Data Processing

Qualitative Data

Potassium Hydrogen Phthalate ( referred to in the experiment as KHP) was a brittle, white, crystalline substance. The crystals required intense stirring before they could be dissolved in water. The resultant Acidic solution was transparent, with a small amount of undissolved granules of KHP. As the transparent NaOH solution came into contact with transparent phenolphthalein in the KHP solution, it turned pink which on shaking became transparent. However, as NaOH was added further, there came a point when no amount of stirring changed the pink colour.

Formulae

Uncertainties

1. % Uncertainty of KHP Mass = (0.01/m_KHP) x 100
2. % Uncertainty of (aq) KHP in Volumetric Flask = (0.1/100) x 100
3. % Uncertainty of (aq) KHP in Pipette = (0.1/10) x 100
4. % Uncertainty of NaOH Volume = (0.1/ V_NaOH) x 100

Stoichiometric Calculations

Moles (n_vf) of KHP in volumetric flask = m_KHP/M_KHP where M_KHP is the Molar Mass of KHP (204.22 g)

[c]_KHP in 100 cm³ volumetric flask = n/V

Moles of KHP in 10 cm³ of solution in where V is a given volume of water

conical flask (n_cf) = [c]_KHP x V

General Formulae

n = m/M mol

[c]₁ x V₁ = [c]₂ x V₂

The volume of NaOH added = Final Volume – Initial Volume

where

n is the number of moles of KHP

m is the mass of KHP in grams

M is the Molar mass in grams

V is the volume in cm³

[c]KHP = (n/V) mol dm^-3 = (0.00974/0.1) mol dm^-3 = 0.0974 mol dm^-3

Where [c]KHP is the concentration of KHP Acid.

Sample Calculations

Number of moles of KHP in 2.00 grams = (m/M) = (2/204.22) mol = 0.00979 mol

[c]_KHP = n/V = (0.00979/0.1) mol dm^-3

Number of moles of KHP in 0.01 dm³ of solution in conical flask = [c] x V

= 0.0979 x 0.01 = 9.79 x 10^-4 mol.

KHC₈H₄O₄ + NaOH → H₂O + NaKC₈H₄O₄

Mole ratio = 1 KHP:1NaOH

From the mole ratio, the number of moles of NaOH = 0.00979 mol.

[c]_NaOH = n/V = (0.00979/0.0950) = 0.103 mol dm^-3 (cm³ is converted into dm³)

Raw Data

Lab 1: Preparation of KHP Acid

Weight of weighing boat before adding KHP = 2.67 g

Weight of weighing boat with KHP = 4.67 g

Weight of weighing boat after transfer = 2.68 g

Molar Mass (M) of KHP = 204.22 g

Volume (V) of water = 100 cm3 = 0.1 dm3

Mass of KHP Transfer = Weight of weighing boat with KHP – Weight of weighing boat after transfer

=    (4.67 − 2.68) g = 1.99 g.

Lab 2: Standardisation of NaOH solution

*Initial volume is the initial reading of the burette and final volume is the reading after adding NaOH solution

Calculations

KHC₈H₄O₄ + NaOH → H₂O + NaKC₈H₄O₄

Mole ratio = 1 KHP: 1NaOH

From mole ratio, number of moles of NaOH = 0.00974 mol

Uncertainty Calculations

Data Analysis and Conclusion

One experimental flaw which resulted in readings inconsistent with the literature value was due to human error. This flaw was due to allowing excess sodium hydroxide to flow, causing the KHP solution to become pinker than it should have. This might have caused some deviations because the volume of sodium hydroxide added was excess. The theoretical value of NaOH to be poured was 9.50 cm³, and more or less than 0.1 cm³ of that value. However, the amount I added on an average was 10.4 cm³, which suggests why the solution became unusually dark pink as supposed to light pink.

The theoretical value of the Sodium Hydroxide that was expected to be used was 9.50 cm³. However, there has been a deviation of 0.9 cm³, which is significant, but not high. The resulting percentage error out of this deviation is:

There is almost a 1% deviation. Taking the value of 9.50 cm³ and mass of 2 grams, the concentration of NaOH should have been 0.103 mol, but the value I obtained due to the excessive deviation gave me 0.0937 mol. The percent error that has resulted in:

9.03% is by far a significant error that has resulted from a small error in the volume. Another error was caused by the deviation in the mass of KHP. 2.00 grams was the amount expected to be taken, but the experimental amount was 1.99 grams. However, this, being only 0.01 grams of the expected value, could have only constituted a very small portion of the error. This means that due to systematic error, my accuracy has fallen by 9.03%, which, although not high, is quite a deviation inaccuracy.

The deviation in the volume, however, is not the only indicator of noticeable systematic errors. The volumes of NaOH used up show significant fluctuations. For example, in trial 1, I used 11.0 cm³ of NaOH, which is 1.50 cm³ off 9.50 cm³, and in my rough trial, the volume used was 9.9 cm³ and in trial 2, the amount used up was 10.4 cm³. The difference between these sets of data indicates that the systematic error of allowing the KHP solution to become too pale resulted in strange fluctuations. These fluctuations caused the 0.95% error.

The percentage uncertainty calculated of the concentration of NaOH was 2.57%, which indicates that the level of precision, although not low, could have been better. The expected % uncertainty that was expected was 0.500%, and the uncertainty I obtained was 0.503%. Also, the % uncertainty of the volume of NaOH was ±1.05%, taking the value of 9.50 cm³. Therefore, due to flaws in raw data values taken from systematic errors, there has been a deviation in uncertainty too, indicating the impact of methodical flaws. The uncertainty of 2.57% indicates that my values were accurate up to within ±2.57%.

Overall, the data obtained, although not completely inaccurate, is not as accurate as it could have been. These errors were avoidable.

Evaluation

William Anderson (Schoolworkhelper Editorial Team)

William completed his Bachelor of Science and Master of Arts in 2013. He current serves as a lecturer, tutor and freelance writer. In his spare time, he enjoys reading, walking his dog and parasailing. Article last reviewed: 2022 | St. Rosemary Institution © 2010-2024 | Creative Commons 4.0

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