A chemist reacts sodium metal (Na) and chlorine gas (Cl) to form salt according to the balanced chemical equation: Na + Cl → NaCl.
If the chemist has 35.0 g Na, what mass of chlorine must he or she use to react completely with the sodium?
Finally, solve (remember significant figures):
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A:
Answers
First we need to convert the 2.34 grams of Cl2 (chlorine gas) to moles of Cl2. To do this, we look on the periodic table for the atomic mass of Cl. The entry for Cl says that the atomic mass of 1 Cl is 35.453. (Remember that the atomic mass is the number with the decimals.) Ok, so we know that the atomic mass of 1 Cl = 35.453. But if we look at Cl2, the Cl has a subscript, and the subscript is 2. (Remember that the subscript is the small number that comes after the element symbol.) So there are 2 Cl in Cl2. So we are going to have to do 2 x 35.453 because there are 2 Cl in Cl2. The calculator says that 2 x 35.453 = 70.906. So the molar mass of Cl2 is 70.906. This means that there are 70.906 grams of Cl2 in 1 mole of Cl2.
Now we need to convert 2.34 grams of Cl2 into moles of Cl2. We set up the equation like this:
grams of Cl2 the problem gave us x 1 mole of Cl2
grams of Cl2
Remember that the problem gave us 2.34 grams of Cl2. We put 70.906 on the bottom (diagonal from the grams the problem gave us). So if we put the numbers in, the equation looks like this:
2.34 grams of Cl2 x 1 mole of Cl2
70.906 grams of Cl2
Notice that we have "grams of Cl2" on the top and the bottom parts. This allows the "grams of Cl2" to cancel out, so we will be left with "mole of Cl2". So to convert, all we have to do is multiply. First we multiply across. 2.34 x 1 = 2.34. Now we divide 2.34 / 70.906 . The calculator says that 2.34 / 70.906 = 0.03300144. Since the "grams of Cl2" canceled out, the unit is now "mole of Cl2". So in other words, we know that there is 0.03300144 mole of Cl2 in 2.34 grams of Cl2. Thus, 2.34 grams of Cl2 = 0.03300144 mole of Cl2.
Now we have found the mole of Cl2. But the problem wants grams of sodium chloride (NaCl). First we need to convert our 0.03300144 mole of Cl2 to moles of NaCl. We look at the balanced chemical equation that the problem gave us: 2 Na + Cl2 --> 2 NaCl . The big numbers in front of the compounds are called coefficients. The coefficient for Na in the balanced chemical equation is 2. The coefficient for Cl2 in the balanced chemical equation is 1. (Remember that if a compound or element doesn't have a coefficient, the coefficient automatically equals 1.) The coefficient for NaCl in the balanced chemical equation is 2.
We use these coefficients in the equation to convert from Cl2 to NaCl. The equation looks like this:
moles of Cl2 x coefficient of NaCl
coefficient of Cl2
In case you can't see this, it is moles of Cl2 x (coefficient of NaCl / coefficient of Cl2). We already found the moles of Cl2. We know that we have 0.03300144 mole of Cl2. We also know that the coefficient of NaCl is 2 and the coefficient of Cl2 is 1. So if we put these values in the equation, it looks like this:
0.03300144 mole of Cl2 x 2
1
If you can't see this, it is 0.03300144 mole of Cl2 x (2/1). The calculator says that 0.03300144 x (2/1) = 0.06600288. So we have 0.06600288 moles of NaCl (sodium chloride).
But the problem wants grams of NaCl. We find the molar mass of NaCl. First, we look at the entry for Na (sodium) on the periodic table. The atomic mass of 1 Na is 22.990. There is no subscript for Na in NaCl, so we only have 1 Na in NaCl. Now we look at the entry for Cl on the periodic table. The atomic mass of 1 Cl is 35.453. There is no subscript for Cl in NaCl, so we only have 1 Cl in NaCl. To find the molar mass of NaCl, we do 22.990 + 35.453. The calculator says that 22.990 + 35.453 = 58.443. So there are 58.443 grams of NaCl in 1 mole of NaCl.
Now we convert our 0.06600288 moles of NaCl into grams of NaCl. We set up the equation like this:
moles of NaCl x grams of NaCl
1 mole of NaCl
Once we put the numbers in, our equation looks like this:
0.06600288 moles of NaCl x 58,443 grams of NaCl
1 mole of NaCl
First we just multiply straight across. 0.06600288 x 58.443 = 3.85741. Now we divide 3.85741 by 1. Of course, 3.85741 / 1 = 3.85741. Since we had "mole(s) of NaCl" in the top and bottom of the equation, the "mole(s) of NaCl" canceled out. We are left with "grams of NaCl". So, we have 3.85741 grams of NaCl. We should round this answer to 3 significant figures because the problem gave us a number with 3 significant figures (the number 2.34). If we round 3.85741 to 3 significant figures, we get 3.86. So, the final answer is 3.86 grams of NaCl (sodium chloride).