Chem 105 Quiz 2

Report this pressure measurement in units of atmospheres: 761.2 mmHg

1.002 atm

1.395 grams of KHP (204.22g/mol) are dissolved in ~30mL of water. A buret is filled with an unknown concentration of NaOH. The initial buret reading is 10.43 mL and the final buret reading is 16.48 mL. Calculate the molar concentration of the NaOH in the buret:

1.13 M

.106 g of KHP was dissolved in ~30mL of water and titrated with an unknown sodium hydroxide solution. The initial volume in the buret is 2.62 mL and the solution turns a pale pink between buret readings of 13.08 mL and 13.94 mL. Calculate the minimum change in volume:

10.46 mL

1.106 g of KHP was dissolved in ~30mL of water and titrated with an unknown sodium hydroxide solution. The initial volume in the buret is 2.62 mL and the solution turns a pale pink between buret readings of 13.08 mL and 13.94 mL. Calculate the ratio of 90%ΔVmin:mass of KHP. You can consider 90% as an exact value

8.512 mL/g

Based on the data in the experiment, is the process of dissolving the following salts endothermic or exothermic: Sodium bromide [A] Potassium hydroxide [B] Ammmonium nitrate [C] Calcium chloride [D]

Specified Answer forA: exothermic Specified Answer for B: exothermic Specified Answer for C: endothermic Specified Answer for D: exothermic

What do all the molecules that show a difference in bond angle in "Real" vs "Model" view have in common

lone pairs

A O2 produced, water level in Pipet A is lowered

true

As the amount of water used in the enthalpy trials decreased, the temperature change of the water increased.

true

Lone pair-lone pair interactions have more repulsion than lone pair-bond pair interactions.

true

The error in the mass% of hydrogen peroxide in the sample should be estimated from the standard deviation of trials 2-4

true

if water level in Pipet A is higher than in B, pressure is lower than atm

true

In the titration of sodium acetate with hydrochloric acid using methyl orange as an indicator, the solution changes from [a] to [b] to [c].

yellow, orange, red

Equations to know for titration lab

ΔVmin = Vbefore color change - Vinitial ΔVmax = Vafter - Vinitial 90%ΔVmin= .9*ΔVmin ratio of 90%ΔVmin:mass of KHP = .9*ΔVmin/mass KHP

electron geometry v molecular geometry

- # of bonding domains -arrangement of bonding domains

titration week 1 errors

- A student forgot to tare the weigh paper when measuring the mass of KHP. (high molarity of NaOH, accuracy is affected) -Some solid KHP spilled when adding it to the Erlenmeyer flask (lower NaOH molarity, not repeatable) -Before titrating KHP with NaOH, the buret was rinsed with DI water and then filled with the NaOH solution (low molarity of NaOH) -When titrating KHP with NaOH, the buret readings were observed from above the meniscus (high molarity of NaOH)

errors that can occur in gas experiment

- Pressure reading is artificially high (higher %m/v) -Temperature reading is artificially low (lower %m/v) -The flask was stoppered 7s after the reaction started (lower %m/v bc O2 was lost) -An excess of KI(s) was added to the flask (nothing) - A 2.00mL sample of H2O2 is added to the flask (greater vol=lower %m/v) -The water levels in the pipets were not equalized at the end of the reaction. The water level in pipet A was significantly lower than that in pipet B (calc results would be lower)

error in heat pack lab

- assuming sausages to be similar to hands -assuming coffee cup is adiabatic - assuming internal temp os sausage to be the same as the one that was measured

gas collection apparatus

- erlenmeyer flaşk, pipet A, pipet B -rxn in erlenmeyer flask produced O2 (g), pushes water out of pipet A into pipet B -pipet B acts as equalizer in rxn, moved to keep water levels the same so that pressure in pipet A is equal to atm pressure -if water level in A is greater than B, P is lower than atm -if lesser in A than in B, P is higher than atm

week 2 titration errors

-Before titrating NaC2H3O2 with HCl, the pipet was rinsed with DI water and HCl. All the solutions are clear and colorless (low HCl molarity, more vol of HCl added, high molarity of sodium acetate) -Before titrating NaC2H3O2 with HCl, all the titrating NaC2H3O2 was forced out of the pipet into the empty beaker (calc molarity of sodium acetate will be greater than it actually is) -When titrating NaC2H3O2 with HCl, the funnel was left in the top of the buret(added drops dec volume change, smaller calc molarity) -When titrating NaC2H3O2 with HCl, the student missed the end point in 2 of the first 3 trials (high vol readings=higher molarity)

use of a catalyst

-H2O2 decomposes slowly, KI used as a catalyst to lower activation energy of rxn and speed it up -catalysts not consumed so don't affect calculated results

gas evolution reaction ex's

-NH4Cl + LiOH(aq) = NH3(g)+H2O(l)+LiCl(aq) -2Al(s) + 6HCl(aq)= AlCl3(aq)+3H2(g) -CaCO3(s) + 2HNO3(aq)= Ca(NO3)2(aq)+ H2O(l) +CO2(g)

equations in gas lab

-PV=nRT -V= water displacement in Pipet A -R=0.08206 -T= room temp -P=atm P -mole of O2= 2 mol H2O2 -(m/v)%H2O2= mass of H2O2/volume of H2O2*100 (g/mL)

adiabatic v closed v open

-adiabatic= no energy/matter exchanged with universe -closed= no matter -open= both energy and matter exchanged

error for titration experiment

-choice of indicator (greater pH range=color change at greater vol=low NaOH molarity) -reading on buret being innaccurate -reaction time in stopping the buret at the color change (late run time= more volume, less calc molarity)

calorimetry concepts

-coffee cup calorimeter ideally is adiabatic -Heat, as a form of energy, can be measured as chemical or physical changes in calorimeters -constant pressure, which indicates that no work is being done to/by the system and that the change in enthalpy, the internal energy of the system, is equal to the heat exchange in the system

secondary standard

-created after being standardized by a primary standard

polarity of molecule

-electronegativity, electron density, shape of molecule

endothermic v exothermic

-endo: absorbs heat, endo rxns will dec temp of soln -exo: releases heat, eco rxns will inc heat of soln

Heat capacity v specific heat

-heat capacity= amt of heat to raise temp of object by one degree C -specific heat= energy needed to raise one gram of substance by one degree C

primary standard

-not hygroscopic (does not absorb water from its surroundings) -high in molecular weight (minimizes error, makes concentration more accurate) -soluble -non reactive -a pure substance -chemically stable

how to calc error

-standard deviation = specific heat of sausage, avg mass of sausage, avg estimate sausage/hand, molar enthalpy of dissolved KOH -error propagation= heat capacity of hand

standardizing

-to determine concentration accurately and precisely

equipment in titration lab

-volumetric pipet (TD) calibrated to deliver 10.00 mL exactly at 25 C, takes up more than 10.00 mL so don't force all liquid out

equations for heat pack lab

1. -q(metal)=q(water) 2. q(sausage)=-q(water) 3.C(hand)=c(sausage)*avg mass of sausage*sausages/hand 4. ΔHKOH=qrxn/mol KOH 5. -qrxn=qsoln (msoln=mwater+msalt) 6. ratio of mass KOH:mass water as conversion factor

1.106 g of KHP was dissolved in ~30mL of water and titrated with an unknown sodium hydroxide solution. The initial volume in the buret is 2.62 mL and the solution turns a pale pink between buret readings of 13.08 mL and 13.94 mL. in another trial, 1.193 g of KHP is dissoved in ~30mL of water. If the initial buret volume is 21.31 mL, at what buret reading should you start adding drop by drop:

31.46mL

leak check

A leak check ensures that the tubes connecting the Erlenmeyer flask and pipets are tightly secured to prevent the produced oxygen gas from escaping, for this can result in a lower calculated mass to volume percent concentration of H2O2 in the solution. This is done by stoppering the Erlenmeyer flask and lowering Pipet B. A small change in the water level in Pipet A is expected, however if the water level continues to change it is indicative that the gas apparatus is not properly assembled.

Which of the following molecules are polar?

CH2O NH3 O3

The errors of which of the following values can be estimated using the standard deviation:

Mass of the average hand Specific heat of Vienna Sausage Molar Enthalpy of dissolving heat pack salt

Which of the following reactions are gas evolution reactions? Select all correct answers.

Na2CO3(s) + HCl(aq) Cd(s) + H2SO4(aq)

molecular equations

NaOH(aq)+KHP(aq)=NaKP(aq)+H2O(l)

Which of the following molecules show a difference in bond angle between "Real" and "Model" view

SO2 SF4 NH3

Fluoromethane (CH3F) is (more/less/equally) [A] polar than chloromethane (CH3Cl), because (fluorine/chlorine) [B] is more electronegative than (fluorine/chlorine) [C].

Specified Answer for: A more Specified Answer for: B fluorine Specified Answer for: C chlorine

choice of indicators

affects accuracy not precision The most accurate indicator that will give the best results for this experiment would be thymol blue. Phenolphthalein would be a poor choice of indicator because it gives the least accurate results.

Before titrating sodium acetate with HCl, for each trial: all the sodium acetate was forced out of the volumetric pipet into the beaker.

calc conc of sodium acetate too low results not repeatable

What type of thermodynamic system is the vacuum insulated tumbler used in this experiment?

closed

What won't result in gas evolution

copper, iodide ion, silver, chloride ion, gold, fluoride ion

endpoint v equivalence point

endpoint= signals the end of titration, signaled by color change equivalence point= moles of titrant equal to moles of analyte, occurs before endpoint -can be achieved at same time if pH is the same for both

First Law of Thermodynamics

energy can neither be created nor destroyed, it can however, be transferred from one location to another or be transformed into different forms

If 0.1g of KI is used, the amount of oxygen gas produced will be less than if 0.5g if used.

false

If the reaction flask containing hydrogen peroxide and potassium iodide is stoppered 7s after the reaction begins, the resulting mass % of hydrogen peroxide in the sample will be too high

false

The units of Heat Capacity are J/gºC

false

as o2 produced, pipet B must be raised

false

rough titration

get a rough estimate of how much added volume is needed before a color change, allows for more time to perform precise titration

Tritration endpoint color using thymol blue

green

Indicators used for pH range of 8.5-9

thymol blue phenolphthalein cresol red

When making the unknown NaOH in 8-1, concentrated NaOH was diluted with DI water, but the solution was not mixed completely.

results not repeatable, calc conc os NaOH too high or too low

lone pairs

take up more space than bonded pairs

analyte v titrant

the unknown thing being analyzed v the solution that is delivered by the buret