CHEMICAL FORMULAE, EQUATIONS AND CALCULATIONS: PART 1 [dchemlc]

what does O₂ represent?

• 2 oxygen atoms joined together

• 2 separate oxygen atoms

• 1 oxygen atom

• 2 oxygen atoms colliding

Answer explanation

2O represents 2 separate O atoms and O₂ means that the atoms are joined together in a molecule.

which of the following is not a state symbol?

• (s)

• (g)

• (aq)

• (ppm)

Answer explanation

what is the unit of relative atomic mass?

• no units

• g

• g/mol

• mol

Answer explanation

The relative atomic mass of an element is the weighted average mass of the isotopes of the element. It is measured on a scale on which a carbon-12 ( 12C) atom has a mass of exactly 12.

Because we are talking here about relative masses they have no units.

what is the relative formula mass of CO₂?

• 44

• 46

• 12

• 32

Answer explanation

what is the mass of 1 mole of oxygen molecules?

• 32g

• 16g

• avagadro's constant

• 8g

Answer explanation

Whenever you talk about a mole of something, you must quote its formula, otherwise there is a risk of confusion.

For example,

1 mole of oxygen atoms, O, has a mass of 16 g

1 mole of oxygen molecules, O₂, has a mass of 32 g.

which of the following equations can be used to accurately convert mass into moles?

• 
• 
• 
• 

Answer explanation

if a compound contains 0.2 moles of magnesium and 0.2 moles of oxygen, what is the formula of this compound?

• MgO

• Mg₂O₂

• Mg_0.2O_0.2

• O_0.2Mg_0.2

Answer explanation

for example;

If we did an experiment and found that a compound contained 0.2 mol Ca and 0.2 mol O, then we could work out that the formula must be CaO because there are the same number of Ca atoms as O atoms.

Copper reacts with concentrated nitric acid according to the equation:

Cu_(s)+4HNO_3(aq)----> Cu(NO₃)_(aq) + 2H₂O_(l) +2NO_2(g) 0.4 moles of copper was reacted with 0.4 moles of nitric acid. Which substance is in excess?

• Cu

• NO₂

• HNO₃

• H₂O

Answer explanation

Looking at the chemical equation we can see that 1 mol Cu reacts with 4 mol HN0₃.

we have 0.4 moles of nitric acid which will react with just 0.1 mole of copper. But we have 0.4 moles of copper. Therefore, copper is in excess.

which of the following equations can be used to find the percentage yield?

• 
• 
• 
• 

A compound has a relative formula mass 30. The empirical formula of this compound, CH3 has a relative formula mass of 15. what is the molecular formula of this compound?

• C₂H₆

• CH₃

• C_0.5H_1.5

• C₃H₈

Answer explanation

relative formula mass of the compound was 30. CH₃ has a relative formula mass of 15(A_r of H = 1, A_r of C = 12).

All you need to find out is how many times 15 goes into 30. 30/15 = 2

So you need 2 lots of CH₃, in other words, C₂H₆.

The molecular formula is C₂H_6.