Short-spined sea urchin fossils were found directly above long-spined sea urchin fossils in a layer of rock. What do the differences between the two sets of sea urchin fossils suggest about them? The longer spines developed as the shorter-spined sea urchins grew older.
The longer-spined sea urchins existed before the shorter-spined sea urchins.
The shorter-spined sea urchins stretched their spines to ward off predators.
The shorter spines did not protect the sea urchins well enough and they died off.

The longer-spined sea urchins existed before the shorter-spined sea urchins

Explanation:

The lower buried is what died off first and the recently buried is the newly evolved version.

1) 0.5 mol.

2) 0.75 mol.

3) 16.8 L.

Explanation:

1) How many moles of Al react?

We can calculate the number of moles of Al (n) using the relation:

n = mass / molar mass,

Mass of Al = 13.5 g.

Molar mass mass of Al = 26.98 g/mol.

∴ The number of moles of Al (n) = mass / molar mass = (13.5 g) / (26.98 g/mol) = 0.5 mol.

2) How many moles of H₂ are produced?

It is a stichiometry problem.We should write the balanced equation of the reaction:

2Al + 6HCl → 2AlCl₃ + 3H₂.

It is clear that 2.0 moles of Al reacts with 6.0 moles of HCl to produce 2.0 moles of AlCl₃ and 3.0 moles of H₂.

Using the cross multiplication:

2.0 moles of Al → 3.0 moles of H₂, from the stichiometry,

0.5 mole of Al → ??? moles of H₂.

∴ The number of moles of H₂ are produced = (0.5 mole)(3.0 mole) / (2.0 mole) = 0.75 mol.

3) How many liters of H₂ are produced?

It is known that at STP, 1.0 mole of any gas occupies a volume of 22.4 L.

Using cross multiplication:

1.0 mole occupies → 22.4 L.

0.75 mole occupies → ??? L.

∴The number of liters of H₂ are produced = (0.75 mole)(22.4 L) / (1.0 mole) = 16.8 L.