The answer to your question is: Chemical Reaction.
Explanation:
In the attachment fig ABCD is a parallelogram with AC as diagonal.
we know that,
In a parallelogram opposite sides are equal and parallel
In a parallelogram sum of adjacent angles is 180°
that is
∠A+∠D= ∠B+∠C= 180°
Also, in a parallelogram opposite angles are equal.
So in ΔADC and ΔABC
1) AD=BC
2) DC= AB
And
3) ∠ADC= ∠ABC
therefore, ΔADC and ΔABC are congruent to each other and Emily was right.
Number of atoms : 1.26 x 10²³
<h3>Further explanation </h3>
The mole is the number of particles(molecules, atoms, ions) contained in a substance
1 mol = 6.02.10²³ particles
Can be formulated
N=n x No
N = number of particles
n = mol
No = Avogadro's = 6.02.10²³
0.21 moles of Al, so n = 0.21
Number of atoms :

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Explanation:
Because molarity is classified as moles of solute per liter of water, dilution of the water may result in a reduction of its concentration.
Therefore, because the amount of moles of solute has to be constant for dilution, you will use the molarity and volume of that same target solution to calculate how many moles of solute will be present in the sample of the stock solution that you dilute.
c = 
⇒ n
=
c
⋅ V
= 0.250 M ⋅ 6.00 L =
1.5 moles HCl
Now all you have to do is figure out what volume of 6.0 M stock solution will contain 1.5 moles of hydrochloric acid
c = 
V = 
=
=
0.25 L
Expressed in milliliters, the answer will be
→ rounded to two sig figs