Moles 2

Question 1

How do you find moles in a gas?

Answer 1

moles = volume (dm³) / 24 (dm³)
1000cm³ = 1dm³

Question 2

How do you find moles in a solution?

Answer 2

moles = concentration (mol/dm³) x volume (dm³)

Question 3

What is a titration?

Answer 3

technique used to accurately measure the volume of one solution that exactly reacts with another solution

Question 4

What is the titre value?

Answer 4

difference between start and end point

Question 5

What equipment is needed in a titration?

Answer 5

volumetric flask
volumetric pipette
pipette filler
burette

Question 6

repeat titrations until …

Answer 6

we get concordant results (within 0.2cm³) → average these results

Question 7

Why is universal indicator not suitable for titrations?

Answer 7

does not give clear colour change (end point)

Question 8

Why is a white tile placed under the flask?

Answer 8

white tile allows clear colour change to be seen ( end point )

Question 9

Why are the same number drops of indicator used each time?

Answer 9

same colour change each time

Question 10

Why should you make sure the gap below the tap of the burette is filled with solution?

Answer 10

starting volume would be incorrect as volume goes down but nothing added to alkali (titre volume incorrect)

Question 11

Why should you swirl the flask as the dilute acid is added?

Answer 11

mix reactants and indicator

Question 12

Why do you repeat the experiment?

Answer 12

to obtain concordant results

Question 13

Why do you add the hydrochloric acid slowly at first then drop by drop near the end point?

Answer 13

to see colour change clearly

Question 14

Titration calculation:
Chloe found that 25cm° of 0.2 mol/dm? sodium hydroxide was needed to neutralise 15cm of hydrochloric acid. Calculate the concentration of the hydrochloric acid.
NaOH + HCI –> NaCl + H20

Answer 14

Moles of NaOH = conc x vol
moles of HCl = ratio in question
conc of HCl = moles/volume

Question 15

Describe how to perform a titration

Answer 15

use the pipette to add potassium hydroxide (solution) to the (conical) flask

add a few drops of methyl orange (to the flask)

add sulfuric acid to the burette and record the initial burette reading

add sulfuric acid (from the burette) to the flask, swirling (continuously)

until the (methyl orange) indicator turns red/orange

take the final burette reading and find volume of acid added (by finding difference between initial and final burette readings)

repeat (the titration) adding the acid dropwise near the end-point / repeat (the titration) to obtain concordant results