There are 3 significant figures in the number 0.00666. They are each of the non-zero number 6's. The zero's before a number are not significant. The zeroes AFTER a number ARE significant.
For example:
0.006660 has four significant figures because there is a zero after nonzeroes.
0.00666 only has three significant figures because the zeros before a significant figure are not significant.
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solids have particles that are closely packed to each other that they can barely move
2.1618 grams of P4 is required to produce 4.21x10^22 molecules of phosphorus trifluoride.
Explanation:
The balanced chemical reaction for formation of PF3 from yellow phosphorus is given by:
P4 + 6F2 ⇒ 4PF3
1 mole of P4 reacts to give 4 moles of PF3
It is given that 4.21x10^22 molecules of PF3 are produced
so the number of moles can be calculated by the relation
number of molecules = number of moles × Avagadro number
number of moles = 
n = 
n= 0.06989 moles of PF3 is formed.
Applying stoichiometry,
1 mole of P4 gives 4 moles PF3
x mole will produce 0.06989 moles of PF3
= 
4x= 0.0698 × 1
x = 
x= 0.01745 moles of P4 will be required.
The weight of the phosphorus can be obtained by number of moles × atomic mass of one mole of P4
= 0.01745 × 123.89
= 2.1618 grams of P4.