C4 chemical reactions

Question 1

How much atoms are there in a mole?

Answer 1

6.022x10^23
(avogadros constant)

Question 2

What is the formula linking moles and mass

Answer 2

n=number of moles
m=mass
Mr=Relative formula mass
Ar =relative atomic mass

n=m/Mr or n=m/Ar

m=nxAr or Mr

Question 3

What do you call reactants after the reaction?

Answer 3

A reaction finishes when one of the
reactants
is all used up. The other reactant has nothing left to react with, so some of it is left over:

the reactant that is all used up is called the
limiting reactant
- it sets a limit on how much
product
can form.
the reactant that is left over is described as being in excess.The mass of product formed in a reaction depends upon the mass of the limiting reactant. This is because no more product can form when the limiting reactant is all used up.

Question 4

How do we find the molar ratio?

Answer 4

In balanced equations
, the number in front of each formula shows the ratio of the
reactants and products. It tells us how many moles of each chemical we need to react to make the products. If there is no number, the amount is one mole.

Question 5

How does the product of the reaction depend on the limiting reaction

Answer 5

The mass of product formed in a reaction depends upon the mass of the limiting reactant. This is because no more product can form when the limiting reactant is all used up.

Question 6

How to find the percentage of an element in a compound by mass?

Answer 6

To find what percentage of a
compound is a particular element, you must first calculate the relative formula mass (Mr) of the compound. Then, use the following equation:

% of an element=mass of element in a compound/Mr x100

Question 7

What is amount measured in

Answer 7

number of moles

Question 8

What is theoretical and actual yield?

Answer 8

Theoretical yield
: the maximum possible mass of a product that a chemical reaction can make. It is calculated using molar ratios.

Actual yield
: the mass of a product that a chemical reaction makes in real life. It is usually less than the theoretical yield, for a number of reasons:
some of the product may be lost when the products are removed from the reaction mixture.
there might be side reactions – unwanted reactions that compete with the desired one.
the reactions may be reversible and may not go to completion. See also Redox, rusting and iron.

Question 9

What is percentage yield?

Answer 9

a comparison between actual yield and theoretical yield.

percentage yield=actual yield/theoretical yield x100

Question 10

What is the empirical and molecular formula?

Answer 10

The
empirical formula of a compound is the simplest, whole number ratio of atoms of each element in a compound.

The molecular formula shows the actual number of atoms of each element present in a compound.

Question 11

Method for Titration

Answer 11

This is an outline method for carrying out a titration in which an acid is added to an alkali. The method is the same for sulfuric acid, hydrochloric acid and nitric acid.

Use the pipette and pipette filler to add a measured volume of sodium hydroxide solution to a clean conical flask.
Add a few drops of indicator and put the conical flask on a white tile.
Fill the burette with hydrochloric acid and note the starting volume.
Slowly add the acid from the burette to the alkali in the conical flask, swirling to mix.
Stop adding the acid when the
end-point
is reached (when the indicator first permanently changes colour). Note the final volume reading.
Repeat steps 1 to 5 until concordant titres are obtained. More accurate results are obtained if acid is added drop by drop near to the end-point. Record the results in a suitable table. Readings should be recorded to two decimal places, ending in 0 or 5 (where the liquid level is between two graduations on the burette). The
titre is the volume added (the difference between the end and start readings).At least two concordant titres should be ticked (✔) in the table above. Concordant titres often lie within 0.10 cm3 or less of each other.Calculate the mean titre from the table above. Ignore the rough run, and run 2 (because they are not concordant):

Question 12

formula to find number of moles in a volume of gas

Answer 12

v= volume in dm^3

n=v/24dm^3

Question 13

why is the actual yield often smaller than the expected yield?

Answer 13

For many chemical reactions, the actual yield is usually less than the theoretical yield, understandably due to loss in the process or inefficiency of the chemical reaction.

Question 14

Why cant you use universal indicator in titrations?

Answer 14

Universal indicator is not a suitable indicator for use in titrations as it only provides a gradual change in colour as the pH of the solution changes. The aim of the titration is to observe the end-point of the reaction which can only be indicated as a single, sharp colour change.

Question 15

What is atom economy?

Answer 15

The
atom economy of a reaction is a measure of the amount of starting materials that end up as useful products. It is important for
sustainable development and for economic reasons to use reactions with high atom economy.

Formula:
Atom economy=(total Mr of desired product/totalMr of all reactants)x100

Question 16

Si2H6 reacts with oxygen.
The equation for the reaction is:
2Si2H6(g) + 7O2(g) → 4SiO2(s) + 6H2O(g)
30 cm3 of Si2H6 is reacted with 150 cm3 (an excess) of oxygen.
Calculate the total volume of gases present after the reaction.
All volumes of gases are measured at the same temperature and pressure

Answer 16

USE MOLAR RATIO 2:7=1:3.5
(volume of oxygen for 30 cm3
Si2H6 = 3.5 × 30)
= 105 (cm3
)
(volume of excess oxygen
= 150 – 105)
= 45 (cm3
)
(volume of water (vapour)
= 3 × 30)
= 90 (cm3
)
(volume of gases = 45 + 90)
= 135 (cm3
)
allowed alternative approach:
(moles Si2H6 =
0.03
24 =)
0.00125 (1)
(moles water vapour formed =
3 × 0.00125 =) 0.00375
and
(moles oxygen used =
3.5 × 0.00125 =) 0.004375 (1)
(moles excess oxygen =
0.15
24 - 0.004375 =)
0.001875 (1)
(volume of gases =
24 × (0.00375 + 0.001875) =
0.135 dm3 =)
135 (cm3
) (1)

Question 17

Ar formula:

Answer 17

Mass of atoms x atoms abundance/ number of atoms