3. Quantitative Chemistry
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Why do chemists need to know how to write and balance equations?
• know the quantity of products • know the quantity and proportions of reactant they’ll need • determine quantity of waste product
What happens to mass during a reaction?
It is conserved
What is relative formula mass?
The sum of the relative atomic masses of each atom in a compound
What can relative formula mass be shortened to?
Mr
Why is there always uncertainty when results are obtained?
Due to errors
How to avoid a measurement error when using a measuring cylinder?
• measure from bottom on meniscus * measure from eye level
What does judging the meniscus from eye level prevent?
A parallax error
What has to be ensured when using a gas syringe so no gas escapes?
That it is sealed tightly
What is a zero error when using a gas syringe?
When the gas syringe has not been set back to zero before taking the measurement
How can a zero error be prevented?
By setting the gas syringe or top pan balance back to zero before taking a reading
What is the result of a zero error?
A measurement appears larger than it actually is
What can disruptions to measurements when using a top pan balance be due to?
• other people (e.g. leaning on table) • wind/breeze • contamination (e.g. already powder on balance)
How can contamination of a top pan balance be avoided?
By cleaning balance with a soft brush
What are some reasons for uncertainty when collecting measurements?
• product/reactant is impure • losses have occurred • sample is wet
How can a product or reactant being impure affect certainty of measurements?
Less than 100% of the recorded mass is what you think it is
How can losses occurring affect the certainty of measurements?
Products like gases can escape easily, and solid powders can be blown away
How can a sample being wet affect the certainty of measurements?
Water can contaminate solids, so they appear heavier
What does the range do?
Represents the distribution of results
How is uncertainty shown using the mean and range?
Uncertainty = mean +/- furthest away figure from mean
How is percentage uncertainty calculated?
Range / Mean x 100
How many atoms, molecules or ions are in 1 mole?
6.02x10 to the power of 23
If you have 1 mole of an element, how much will it weigh?
The relative atomic mass in grams
How many atoms are in 12g - its relative atomic mass - of carbon?
6.02x10 to the power of 23
How is mass calculated? (Mr and mol)
Mr x Mol