Chemistry 2 Lab Midterm

sample average

Add all the numbers of the data set then divide by the amount of data you collected. (x̄)

sample variance

s²=1/(n-1)∑(xᵢ - x̄)²

sample standard deviation

The square root of the sample variance, representing the dispersion of a data set. s=√(s²) = √(1/(n-1)∑(xᵢ - x̄)²)

the larger the standard deviation means what?

a greater dispersion or variability in the data set, which means more uncertainity in the measurements and a lower precision.

Confidence Interval

CI=x̄±(t*s)/sqrt(n)

absolute error formula

The difference between the measured value and the true value of a quantity, indicating the accuracy of the measurement. Absolute Error = |Measured Value - True Value|

Relative error

%e_x=|absolute error|/measured quantity = e_x/x. Relative Error is a measure of the accuracy of a measurement, expressing the absolute error as a percentage of the measured quantity.

Relative error as a percent

the absolute error divided by the measured quantity and multiplying by 100.

Addition and subtraction propagated error formula

e_z=±sqrt(e_x²+ e_y²)

Addition and subtraction propagated error formula relative error

e_z= ±sqrt(ex² + ey²) / z

Error propagation for multiplication and division

Suppose the desired value z is obtained from the multiplication of z=xy. %e_z=(e_z/z)=sqrt([e_y/y]+[e_x/x])

For y=x^a

the relative error is given by %e_y = |a|*%e_x.

For y=ln(x)

the relative error is given by e_y = e_x/x= %e_x.

For y=b^x

e_y/y=ln(b)e_x

For y=log_bx

e_y=1/(ln(b))(e_x/x)

For y=e^x

the relative error is given by e_y/y = e_x

For pH calcuations, what error propagation formula do you use?

For pH calculations, use e_[pH] = 0.434 × (e_H+ / [H⁺])

error propagation shortcuts for disproportionate error terms

ONLY USE WHEN e_x ≤10e_y. z=xy, e_z=z/y(e_y) | z=x/y, e_z=z/y(e_y) | z=x+y and z=x-y, e_z=e_y |

Dalton’s Law of Partial Pressures

P_T= P1 + P2 + P3 + … + Pn, where each P represents the partial pressures of individual gases in a mixture.

How to calculate gap in Q test

absolute value of the outlier minus the next one in the sequence

How to calculate the range in a Q test

largest minus smallest.

How to know whether to reject an outlier.

Q_exp>Q_crit .

How to calculate the new standard deviation in a Q test

Divide the range by the square root of the sample size minus one.

TC glassware

Used when the total volume of a liquid is required, such as in volumetric flasks or graduated cylinders for making solutions. TC (To contain).

TD glassware

Used for transferring specific volumes from one vessel to another. TD (To deliver)

Dry gas formula

P_{dry gas}=P_total-P_HOH

Parallax

is an error that occurs when reading measurements due to the observer's angle of sight, which can lead to inaccurate readings in laboratory instruments.

Height of water column values

P_atm=P_HOH+dgh

Buret readability error

±0.02 mL

Which type of soda density would be closer to water density, diet or regular?

Diet soda density is closer to water density because it typically has fewer dissolved solids compared to regular soda.

Experiment 1 Summary

Using an apparatus with determined values of P, V, n, and T, we can then determine R. Decompose some salt with heat in the presence of an oxide. Because, when oxygen is produced it pushes an equal volume of water out of the flask and into the beaker, so measuring the volume of water displaced into the beaker gives you the volume of O₂ produced.

Experiment 2 Summary

Determining the density of soda to see if it is diet or regular. A density closer to 1 means that it is closer to water density, which means it is diet.

Experiment 3 Summary

Use chromatography to see what substances flow out first based on teh charge of the slush inside of the buret. Anion chromatography means we separate negatively charged ions, while cation chromatography separates positively charged ions.

Experiment 4

Prepare a standardized NaOH solution and titrate correctly.

Experiment 5

Create a titration curve.

Slope of y=11154x+0.0245

εb in Absorbance/M. b=1cm.

Intercept of y=11154x+0.0245

This is the baseline absorbance when concentration = 0.

Ideally this would be 0, but in reality there is always some background absorbance from:

• solvent

• cuvette imperfections

• instrument noise

• incomplete blanking

y and x in y=11154x+0.0245

y is the absorbance. x is the concentration in the cuvette.

Experiment 3 Formula

Cr₂O₇^2-+6Fe²⁺+14H⁺—> 2Cr3+ + 6Fe3+ + 7H2O

what elutes first in a CATION exchange chromatography, cations or anions

anions.

what elutes first in a ANION exchange chromatography, cations or anions

cations elute first

IF a molecule has two COOH groups, what can you assume of it?

It is diprotic.

Monoprotic

can only donate one proton

Diprotic

can donate two protons (H+) in solution.

Amphiprotic

can act as both an acid and a base, capable of donating or accepting a proton.

Why do you never blow air into a volumetric pipet?

Blowing air into a volumetric pipet can introduce bubbles, which leads to inaccurate volume measurements and compromises the precision of liquid transfer.

Why do you condition burets and pipets?

Conditioning burets and pipets ensures that they are clean and prevent contamination, allowing for accurate and reproducible measurements by rinsing them with the solution they will hold.

What are the correct posture rules for buret viewing?

Avoid parallax. Read to the nearest 0.02 mL. Ensure the buret is at eye level. Manipulate stopcock. (Open stopcock is vertical, closed is horizontal)

3 rules to improve the accuracy of a measured value

1. If a cause is known for an error, discard the result.

2. If there are three or fewer results in a set, do not discard any results except for by case 1.

3. Apply the Q test.

How do you know which Ka to use when provided an amphiprotic substance?

pKa1 is the pka for the first dissociation step: H₂A → HA^- + H⁺

pKa2 is the pKa for the second dissociation step: HA- → A^2- + H+.

What are the prerequisites for a good primary standard?

A good primary standard must be pure, stable, and not hygroscopic. It should have a known high molar mass and be readily available.

On a cooling curve, where can you find Tf?

Tf is found at the left most side of the line at t=0.

What are the steps to calculate the molarity of an unknown solution using a cooling curve?

First, use Q=mCsdT. Assume 100g of solution total, and Cs=4.184. Then use dH=Q/mol LR. Then divide by the total volume.

How do you know when to use the Nernst equation to calculate the half cell potential?

When Q=/=1

What directions do currents flow in a galvanic cell?

Currents flow from cathode to anode

When a red lead is connected to what end is the voltage positive?

The highest potential terminal (cathode)

How can you predict whether a reaction will oxidize/reduce something under standard state conditions?

When dE > 0 then the reaction is spontaneous. Compare and see if it is greater than zero. After that, right the complete equation.

How do you find the rate of the iodine clock target reaction?

R= 0.5 [S2O3]i / dT

For the iodine clock reaction, how do you know and can predict elapsed time it will take?

However much of the active ingredient being consumed is put into the solution. (THE LIMITING REAGENT) If someone uses less of S2O3, than the elapsed time in the experiemnt will be the shortest. If you have a larger volume than the concentration of solvents decreases, so it takes longer for the reaction to occur.

For the iodine clock reaction, how do you know and can predict the rate?

For the rate, it depends on concentration. More volume means less concentrated solution and slower rate.

Will the rate constant ever be affected by concentration or elements?

No, the rate constant is independent of concentration and only changes with temperature and nature of the reaction.