When 38.0 ml of 0.1250 m h2so4 is added to 100. ml of a solution of pb(no3)2, a precipitate of pbso4 forms. the pbso4 is then filtered from the solution, dried, and weighed. if the recovered pbso4 is found to have a mass of 0.0471 g, what was the concentration of nitrate ions in the original solution, assuming that all of pb2+ has been precipitated?
a) 1.55 × 10–3 m
b) 3.11 × 10–3 m
c) 1.55 × 10–4 m
d) 6.20 × 10–3 m
e) 3.10 × 10–4 m

b) 3.11x10⁻³ M

Explanation:

We have:

38 ml H2SO4 0.1250 M

100 ml Pb(NO₃)₂

The chemical equation for this case is:

H₂SO₄ (ac) + Pb(NO₃)₂ (ac) →  PbSO₄(s) + 2HNO₃(ac)

Sentence says that, once the solution of H₂SO₄ was added to the Pb(NO₃)₂ solution, we obtain a precipitate of PbSO₄ that finally weights 0.0471 g

First, we should obtain the molecular mass of this precipitate:

Pb= 207 g/mol

S= 32 g/mol

O= 16 g/mol

These values are obtained from Periodic Table, then:

Molecular mass of PbSO₄= 303 g

303 g PbSO₄=1 mol

0.0471 g PbSO₄= 1.55x10⁻⁴ mol

According to the stoichiometry of the above reaction, with 2 moles of NO₃⁻, we get 1 mol of PbSO₄, so:

1 mol PbSO₄=2 moles NO₃⁻

1.55x10⁻⁴ mol PbSO₄=3.11x10⁻⁴ moles NO₃⁻

Finally, we must obtain the concentration of these ions, in 1 Lt (1000 ml):

100 ml=3.11x10⁻⁴ mol NO₃⁻

1000 ml (1Lt) = 3.11x10⁻³ M NO₃⁻

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it can be observed without changing the identity of the object.

it depends on what substance you're talking about as well as many other factors. in quantum mechanics, the observation of particles affects the observers ability to measure certain properties. for example: you can know the velocity of a particle, but you can't know it's position; or you can know it's position, but not it's velocity. it's like trying to find out where a grain of sand is by throwing other grains of sand at it. once you find out where it is, it's been moved. it's also like trying to find an answer to the question of whether or not a tree falling in a forest makes a sound if nobody is there to hear it. paradoxically, the only way to find out is to be there and hear it.