C4- calculations➖➕➗✖️

Question 1

RECAP: What’s an isotope?

Answer 1

Isotopes are: atoms of the same element, with a different number of neutrons.

Question 2

What is the difference between relative formula mass and relative atomic mas?

Hint: add all of the ________ atomic mass of each _______, in the compound.

Answer 2

• relative atomic mass is always the biggest number on the periodic table (when we look at an element). We don’t have to calculate anything. Think m, for massive.
• However, with relative formula mass, you need to calculate it, using their periodic table. And by adding all of the relative atomic mass of each element, in the compound.

Question 3

What is relative formula mass?

Hint: adding

Answer 3

• With relative formula mass [Mr], you need to calculate it, using their periodic table.
• And by adding all of the relative atomic mass of each element, in the compound.

Question 4

What is relative atomic mass?

Answer 4

• relative atomic mass [Ar] is always the biggest number on the periodic table (when we look at an element).
• We don’t have to calculate anything. Think m, for massive.

Question 5

What’s the equation for percentage by mass equation?

Hint: x no of _____ in that element.

Answer 5

• Ar of element in compound x no of atoms of that element/Mr of compound.
• Then multiply by 100

Question 6

RECAP: What’s the equation that links number of moles, mass and relative formula mass?

Answer 6

Number of moles= Mass
_____________
Mr

Question 7

What is Avogrado’s constant? And what’s it used for?

Answer 7

6.02x10^23

• Avogrado’s constant is used to calculate the no. of atoms or molecules.

Question 8

Fill in the gaps:

• When we use the number of _____ , mass and Mr equation we might have a single element, instead of a ________
• If this is the case, only use the relative ______ mass of that _______.

Answer 8

• When we use the number of moles, mass and Mr equation we might have a single element, instead of a compound.
• If this is the case, only use the relative atomic mass of that element.

Question 9

What is the conservation of mass?

Hint: atoms

Answer 9

• mass is always conserved in a chemical reaction because:
• In a chemical reaction, no atoms are created or destroyed. Only the bonds between atoms change.

Question 10

What is the exception to the conservation of mass?

Hint: might seem to change because… , float and come from

Hint#2: scales take into account

Answer 10

• when a reaction involves a gas, the mass might seem to change because the gas can come from the air, or float of into the air.
• [our scales might not also weigh all the reactants, because it didn’t take into account the gas]

Question 11

In an equation, what side is the products and the reactants?

Answer 11

The reactants are on the left side + the products are on the right.

Question 12

If an experiment was to be done in a sealed container, and measured. What will it take into account?

Hint: the gas and

Answer 12

• Because the gas would be trapped in the container, when the reactants’ mass are measured, it would take into account: the gas and the solid.

Question 13

Why might a mass seem to increase?

Hint: reactants

Answer 13

• the mass may seem to increase because one of the reactants is a gas, that’s found in the air.
• And all the products are: solids, liquids or aqueous.

Question 14

Why might a mass seem to decrease?

Answer 14

• the mass may seem to decrease, because one of the products is a gas.

Question 15

Referring to reaction vessels, explain why a mass may seem to increase. [3]

Hint: floating, part of the product, inside

Answer 15

• before reaction, gas is floating around in the air and it is not contained in the reaction vessel- meaning you can’t account for its mass.
• ; when the gas reacts to form, part of the product, it becomes contained inside the reaction vessel.
• this means the total mass of the ‘stuff’ inside the reaction vessel increases.

Question 16

Referring to reaction vessels, explain why a mass may seem to decrease. [4]

Hint: before, isn’t enclosed, gas expands, no longer contained.

Hint#2: can’t account for…

Answer 16

• Before a reaction, all the reactants are contained in the reaction vessel.
• But if vessel isn’t enclosed, the gas can escape from the reaction vessel as it’s formed.
• This is because a gas expands to fill a container it’s in, so if
• it’s no longer contained in it’s reaction vessel, gas expands out from the vessel + escapes into air around——> means you can’t account for its mass + total mass of reaction vessel decreases, if it’s not sealed.