Amount of substance Flashcards
What is Relative Atomic Mass? (Ar)
⇒ (Average/mean mass of one atom of the element) / (1/12th mass of an atom of 12C)
What is the relative molecular mass? (Mr)
⇒ (Average/Mean mass of one molecule) / (1/12th mass of 12C)
How do you find the number of atoms in a substance?
⇒ number of atoms = moles x 6.022x10^23
How do you find the number of molecules in a substance?
⇒ number of molecules = moles x 6.022x10^23
How would you work out the mass of an isotope/element, given the Avogadro’s constant? (1)
⇒ MASS = Mass No1 / Avogadro’s constant
e.g: ⁷⁹Br⁺
Mass = 79/6.022 x 10²³ = 1.31 x 10⁻²² grams
Kg = 1.31 x 10⁻²²/1000 = 1.31 x 10⁻²⁵ kg
What is the formula for concentration?
⇒ Concentration(mol dm^-3) = mol/volume(dm^3)
What is the formula for concentration in terms of mass?
Concentration(g/dm^3)=mass(g)/volume(dm^3)
1dm^3= xcm^3
⇒ 1000
How many grams are there in 1 tonne?
1 tonne = 1000000grams (1x10^6)
How many milligrams (mg) in one 1 gram?
⇒ 1000mg ↔ 1gram (mg↔g= ÷1000)
What is the ideal gas equation? state its SI units.
⇒ pV=nRT → p= pressure, Pa → v= volume, m^3 → n= number of moles, mol → R= gas constant → T= temperature, kelvin
How can you convert cm^3 to m^3?
/1000000 (x10^6)
How would you convert kPa and MPa into pa?
⇒ kPa= x1000
⇒ MPa= x1000000 (x10^6)
1 litre=?
⇒ 1000cm^3 ⇒1dm^3
What is the equation for density?
and its units
Density=mass/volume
⇒ Density (g cm^-3), (Kg M^3)
⇒ Mass (g/kg)
⇒ volume (cm^3), (m^3)
What is the general method of engineering a formula?
⇒ Criss-cross method.
→ take into account the charges and use the criss-cross method to make and overall charge of zero.
eg: k=+1, so4=-2
→ K₂SO₄
Write out the formula for iron(III) hydroxide.
Fe(III)=Fe³
Hydroxide= OH−
⇒ Fe(OH)₃ (compounds need it own individual bracket as you are mutiplying the entire bracket)
What are the Common Positive Ions:
⇒ Zn²⁺ (Zinc)
⇒ Ag⁺ (Silver)
⇒ H⁺ (Hydrogen)
⇒ NH4⁺ (Ammonia)
What are the Common Negative Ions?
⇒ NO₃⁻ (Nitrate) ⇒ OH⁻ (Hydroxide) ⇒ HCO₃⁻ (Hydrogencarbonate) ⇒ CO₃²⁻ (Carbonate) ⇒ SO₄²⁻ (Sulphate)
Define empirical formula.
⇒ The SIMPLEST whole number ratio of atoms of each element in a compound.
Define molecular formula.
⇒ The ACTUAL number of atoms of each element in a compound.
How would we calculate the molecular formula value, needed to give the actual number of atoms, if given the relative molecular mass of the compound?
Relative molecular mass (given in exam question)/Mr of empirical formula
the relative molecular mass of a compound is 284 g mol⁻¹. Given that the compound is P₂O₅. Deduce the molecular formula.
Mr of P₂O₅: (31x2) + (16x5) = 142
284/142 = 2
⇒ 2(P₂O₅) = P₄O₁₀
Give 4 reasons to why 100%yield may not be achieved? (4)
⇒ the reaction is reversible
⇒ there are side reactions occurring
⇒ some of the product is lost when separated or through a gas.
⇒ the reaction might be incomplete.
What is the formula for percentage atom economy?
⇒ (Mr of desired product/Mr of all products) x 100
The formula for & yield:?
⇒ (actual yield/theoretical yield)x100
What can a company do to maximise profits when a reaction has a low atom economy, without changing the reaction conditions or production costs?
⇒ Sell the product(s) to industries for financial gain.
Why do companies in the chemical industry try to use processes with high atom economies? (4)
⇒ Producing little to no waste = better for the environment.
⇒ Less natural resources are used so less energy used
⇒ Less expensive
⇒ Spend less on separating the desired products from waste products and treating waste.
State why it’s important to fill the space below the tap in the burette before beginning an accurate titration (1)
⇒ make sure an incorrect volume is not recorded (1)
State why rinsing the inside of the conical flask with distilled water during a titration can improve the accuracy of the end point (1)
⇒ Any reagents that are stuck on the side of the flask/left over can be added to the mixture.
State why rinsing doesn’t change the volume of solution needed in a titration or the titre value (1)
⇒ Water isn’t an acid or alkali so will not change the moles of the solution/reagent.
A student rinses a BURETTE with deionised water before filling it with sodium hydroxide solution.
State and explain the effect, if any, that this rinsing will have on the value of the titre (2)
⇒ Titre value would increase (1)
⇒ Because the sodium hydroxide solution would be more dilute (1)
Suggest one reason why a 250cm^3 conical flask is preferred to a 250cm^3 beaker for a titration ? (1)
⇒ Reduces the chance of spills when swirling the solution.
Suggest why repeating a titration makes the value of average titre more reliable (1)
⇒ Titrations can give anomalous results, these can be reduced.
Why should small amounts of a substance not be used? (1)
⇒ there might be errors in weighing would be too high/percentage errors too high.
Why would large amounts of a substance not be used? (1)
⇒ There would be an incomplete reaction/decomposition.
State a suitable amount of indicator solution that should be added to the flask (1)
⇒ 2-6 drops/ a few drops.
State 3 ways that could improve the overall technique when carrying out a titration (3)
⇒ 1. Wash the glass rod into the solution to transfer any remaining substance on it. (1)
⇒ 2. Ensure the bottom of the meniscus is on the graduation mark. (1)
⇒ 3. invert the volumetric flask 2 times. (1)
What is the hazard and precaution of using an acid?
⇒ Can be corrosive and harm the skin and eyes if in contact: wear safety goggles/gloves.
What is the hazard and precaution when using a toxic substance?
⇒ Toxic vapours can be inhaled; carry out experiment in a fume cupboard.
Suggest why the student should use NaOH solution rather than water for the final rinse of the burette.
⇒ Water will dilute NaOH and change the concentration.
What are concordant titres?
⇒ Titres that are within 0.1cm3 within each other.
What’s the formula for percentage uncertainty?
% uncertainty = (uncertainty/measured value)x100
e.g: The resistance of a filament lamp is given as 5.0 ± 0.4Ω. Calculate the percentage uncertainty
→ (0.4/5.0)x100 = 8%
Weighing container with solute:10.13g
Weighing container AFTER solute added to volumetric flask: 4.48
EACH READING from the balance has an uncertainty of ±0.005g.
What is the percentage uncertainty in the mass of the solute used? (1)
% uncertainty = (uncertainty/measured value)x100
⇒ (0.005/(10.13-4.48))x100 = 0.09
→ Weighing scale used twice: 0.09 x 2 =0.18% (“after”)
What is the dilution equation?
M₁V₁=M₂V₂ M₁: initial molarity V₁: Initial volume M₂: Final molarity V₂: Final volume
A sample has 2.0mol dm⁻³ acid has a volume of 100cm³
What volume of water, in cm³, should be added to this acid to dilute the sample to a concentration of 1.5mol dm⁻³? (1)
C₁V₁=C₂V₂ → 2 x 100 = 1.5 x V₂ V₂ = (2 x 100)/1.5 = 133.3333333333... → The final volume = 133.333333333333 → Initial volume = 100 Volume added = 133.333... - 100 = 33.3333333
A student identified the use of the burette as the largest source of uncertainty in the experiment. Suggest how the procedure could be improved to reduce the %uncertainty in using the burette. (2)
⇒ Use a larger mass of solid (1)
⇒ So a larger titre reading will be needed (1)
A student is required to make 250cm³ of an aqueous solution that contains a known mass. Describe the method, including apparatus and practical details, that the student should use to prepare the solution. (6)
⇒ : transfers known mass of solid:
→Weigh the sample bottle containing the solid on a
balance
→Transfer to the beaker and reweigh sample bottle
→Record the difference in mass (2)
⇒: Dissolves in water
→Add distilled / deionized water to the beaker
→Stir, until all solid has dissolved (2)
⇒ : Transfer washings
→ Transfer to volumetric/graduated flask with 250cm3 mark, with washings (2)
→ Make up to 250cm3 / mark with water
Shakes/inverts/mixes
