Practical techniques Flashcards
Why would using a 25cm3 measuring cylinder to measure out 150cm3 of acid, have a greater % uncertainty than a vessel e.g. 250cm?
Repeated use of the small measuring cylinder will lead to greater transfer losses.
Why should you filter the solution once solid has been dissolved when obtaining a pure crystal ?
To remove any insoluble impurities by filtration
After solid is allowed to crystallise why are crystals then filtered from the main solution?
To remove any soluble impurities behind after filtering.
Why are the crystals washed with cold water?
So no soluble impurities form on the product as it dries
Describe how impurities affect the melting temperature of an impure solid. [2]
Melting point less sharp/over greater range
Therefore melting temperature less than for pure solid
What is distillation usually used to achieve?
To make or purify an organic liquid
What does steam distillation do?
Steam distillation lowers the boiling point of an organic liquid
what is the appearance of NO2(g)
brown fumes
Why can F- ions not be detected using silver nitrate solution?
silver fluoride is soluble/ won’t form precipitate.
Why is nitric acid needed to acidify the samples in the halide test?
to prevent other precipitates forming in the test
How can you find the Mr of a volatile liquid?
By vaporising a known mass at a known temperature and pressure. Volume measured and then ideal gas equation can be used
Explain what heating to constant mass involves. [2]
keep heating and re-weighing the reactant until mass stops changing
Main problems with Calorimetry?
Heat loss so better insulation can help
Also incomplete combustion of fuel or losing some fuel by evaporation
Why would you want to use a pipette rather than a measuring cylinder?
pipette offers higher resolution (+-0.1 cm3) than a measuring cylinder (+- 1 cm3)
Which pieces of apparatus require 1 reading/measurement?
pipette
measuring cylinder
temperature
volumetric flask
