Tough question, I would suggest using Google maybe, or just plain out asking your science teacher.
Via the half-life equation we have:
Where A is the amount initially and final amount after some time has passed, t is the time elapsed, h is the half life time and so:
Therefore there will be 2.8125 grams left after 140 seconds.
1) B, Not 100% sure though
2)Building a rocket is a physical change technically its not changing its composition its just changing its appearance, the fuel for it causes a chemical change, there is an answer that is more clear, Grinding chalk into a fine powder only changes its shape which is a physical change.
Answer: Correct option is B)
The standard enthalpy change of formation of a compound is the enthalpy change which occurs when one mole of the compound is formed from its elements under standard conditions. The equation showing the standard enthalpy change of formation of water is
H
2
(g)+
2
1
O
2
(g)⟶H
2 ↑
O(l) 1 mole of water formed.
∴ Enthalpy of formation is −X2kJ/mol.
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Explanation:I hope this helped
<span>(3) Atomic masses are a weighted average of the naturally occurring isotopes.
</span>
Here, we found this answer my looking at the key words 'naturally occurring isotopes'
An isotope is a different amount of neutrons, making the atomic mass change.
