Chemical Formulae and Equations

Definitions & True or False

100

Formula for relative molecular mass

M_r = A_r1 +A_r2..etc.

200

Relative molecular mass is found on the periodic table.

False

300

Define M_r

The sum of all the relative atomic masses of the atoms present in a molecule. 

400

Masses of the elements are obtained from the Periodic Table

True

500

Define relative atomic mass (A_r) 

The average mass of naturally occurring atoms of an element on a scale where the carbon-12 atom has exactly 12 units. 

Calculation questions

100

If 30g of Sodium reacts with 50g of Calcium to produce.. (Hint : Mass of Products ) 

80 g of Sodium Chloride 

200

Calculate the relative molecular mass of SO₂

1S = 1 x 32 = 32

2O = 2 x 16 = 32 

Final answer --> 64

300

Calculate the relative molecular mass of calcium carbonate (CaCO₃)

1Ca = 1 x 40 = 40

1C = 1x12 = 12

3O = 3 x 16 = 48

Final answer --> 100

400

Calculate the relative molecular mass of Ammonium sulfate (NH₄)₂SO₄ 

2N = 2 * 14 = 28

8H = 8 * 1 = 8

1S = 1 *32 = 32

4O = 4 *16 = 64

Final answer --> 132

500

 How can you calculate the relative molecular mass of a molecule in steps and give an example?

Here's how to calculate the M_r (Relative Formula Mass) of a compound:

1. Identify the elements and their atomic masses (A_r):

You'll need the chemical formula of the compound and a periodic table or reference with atomic masses. The atomic mass (A_r) of an element represents the average mass of its isotopes and is usually listed on the periodic table.

2. Count the number of atoms of each element:

Look at the subscript numbers in the chemical formula. These indicate the number of atoms of each element present in one molecule of the compound. If there's no subscript, it's assumed there's one atom of that element.

3. Multiply the atomic mass by the number of atoms for each element:

For each element in the compound, multiply its atomic mass (A_r) by the number of atoms you identified in step 2.

4. Add the products from step 3:

Sum the values you obtained in step 3 for all the elements in the compound. This final sum represents the M_r of the compound.

Here's an example:

Calculate the M_r of water (H₂O):

1. Elements and Atomic Masses:

   • Hydrogen (H): A_r = 1.008
   • Oxygen (O): A_r = 15.999

2. Number of Atoms:

   • Hydrogen (H): 2 atoms
   • Oxygen (O): 1 atom

3. Multiply A_r by number of atoms:

   • Hydrogen: 1 x 2 = 2
   • Oxygen: 16 x 1 = 16

4. Sum the products: M_r (H₂O) = 2 + 16 = 18

Therefore, the Relative Formula Mass (M_r) of water (H₂O) is 18.

Randomized

100

A  single carbon 12 isotope is equal to 12 units , what is the A_r of 1 hydrogen.

1 unit 

200

What is the relative molecular mass for NaCl?

58.5

300

Where is the A_r found in the periodic table?

Under the elements symbol

400

What is the type of bond between Li2O?

Ionic bond

500

What is the relative molecular mass for MgO?

a. 80

b. 40

c. 20

b. 40