lead yes, iron and mercury no.

Basically, if the elemental composition stays the same as per the Law,

The percentage mass of the metal will always be the same.

Hypothetical example :

Let's assume that when we burn lead in oxygen, we always get lead(IV) oxide.

Pb + O2 → PbO2

Ar of Pb = 207g/mol
Ar of O = 16g/mol

Mr of PbO2
= (207 + 16 × 2)g/mol
= 239 g/mol

% by mass of Pb in PbO2
= 207/239 × 100%
= 86.6%


So no matter what mass of Pb you burn,

when the burning is complete, the Pb is always going to be 86.6% of the sample's mass.


If you burn 207g of Pb, you're gonna get 239g of PbO2

If you burn twice of that, you're going get twice of the PbO2

The proportion/percentage never changes.

So let's compare the % composition by mass for iron, lead and mercury.


For 20 g of iron, 28.6 g of iron oxides were obtained (pretend we don't know the formula just like Proust didn't before chemical formulas concept was developed)

% composition by mass for iron in the oxide formed

= 20/28.6 × 100%
≈ 69.93% (4s.f)
= 69.9% (3s.f)


For 30 g of iron, 42.7 g of iron oxides were obtained.

% composition by mass for iron in the oxide formed

= 30/42.7 × 100%
≈ 70.25% (4s.f)
= 70.3% (3s.f)


So the two values are very close.


(The difference is about 0.32% points)


Now it is possible that both FeO and Fe2O3 could be formed.


We can check the % composition of Fe in FeO and in Fe2O3

Ar of Fe = 56g/mol

% mass composition of Fe in FeO
= 56/(56 + 16) × 100%
= 77.7% (3s.f)

% mass composition of Fe in Fe2O3

= (56×2)/(56×2 + 16 × 3) × 100%
= 70%

We would say the % composition is closer to FeO.

Do the same for lead and you will get :

92.93%
92.87%


Pretty close, so we might say that PbO (lead(II) oxide) is always formed when lead metal is burnt in oxygen.

Difference is about 0.06% points only. Much smaller than iron's.

(Btw, % composition by mass of Pb in PbO = 207/(207+16) × 100% = 92.8% (3s.f), based on the values of the periodic table used in O levels. If you use more accurate values from other periodic tables, this might differ a little)



For mercury,

92.59%
96.15%

Not close even. Does not obey the Law.


We know now in modern times that both mercury(II)oxide (HgO) and mercury(I) oxide are formed (Hg2O)

how do we know that FeO and Fe2O3 can be formed tho
when elemental composition is the same, does that mean the amount of Fe and O used is the same for both compounds?

does heated in air to a constant mass means that the metal is heated to the point the mass of the metal will no longer increase?

Wait, I'm editing.

Composition can mean by mole ratio, or also by mass ratio.

In H2O, the ratio of H to O is always 2 : 1
The

If we look at mass composition %, the oxygen is always 88.88% by mass.

Heated to constant mass just means the reaction is complete.

All the metal has reacted with oxygen to form metal oxides.

No more reaction occurs and so the mass stays constant.

thx :) i understand it now

by the way what does negative acceleration signifies?

Should be slowing down

thx :) by the way is there any way to understand the physical properties of alkali metals, halogens, transition metals and noble gases? like why do they hv low/high mp and bp and why does it increase/decrease down the grp. also, why do tjey hv high/low density and why do they increase/decrease down the grp

im trying to memorise it but i can't remember it well and often get mixed up as i don't understand the reason behind it...

Maybe you'll have to state which reasons you don't understand first

i dont understand why do the respective metals and gases (the ones i mentioned above) have their physical properties like low/high mp, bp, density and the trends of it

basically the chapter of periodic table... also found it difficult to understand depletion of ozone layer

Alkali metals have lower m.p/b.p because :
Each atom only has 1 valence electron. So the metallic bonding (i.e electrostatic attraction between positively charged nuclei and sea of free mobile electrons) is much smaller in extent compared to something like aluminium (3 valence electrons per atom)

b.p/m.p gets lower as you go down the group because there are more electron shells, so the valence electrons are further away from the nucleus (the atomic radius becomes bigger), so the electrostatic attraction is also weaker.

Even though the number of protons increases, this is outweighed by the higher number of electron shells (which have a shielding effect on the valence electrons) so the effective nuclear charge is lower.

halogens and noble gases have many valence electrons but they also have low melting and boiling point tho. furthermroe their mp and bp increases down the grp...

those two groups you mentioned :

When you talk about mp/bp, you're talking about the separation of individual molecules.

Eg. Increasing the spacing/distance between Cl2 molecules.

Or

if we are talking about atoms for noble gases,

Eg. Between one He and other He atoms.

That's overcoming the intermolecular forces of attraction / Van der Waals' forces / instanteneous dipole-induced dipole interactions.


These forces are much weaker than metallic bonding, which are electrostatic attractions between positive nuclei (i.e the protons) and sea of free mobile electrons which extend the throughout the metal (solid state the atoms are tightly packed in a regular arrangement. For liquid state the atoms are less tightly packed, more disorderly and can slide past each other)

Be careful not to conflate the two.

As for m.p/b.p increasing down the group,

the increase in the number of electrons (and shells) means that there is a greater perturbation or distortion of the electron cloud by the other atom/molecule's nuclei.

This means a stronger induced dipole so the intermolecular forces are stronger.