Number of Moles and Balancing Chemical Equations

[Madison Rhynhart 3H]

Dr. Lavelle indicated that due to the law of conservation of mass we had to make sure that the total number of atoms on the left = that on the right however, does this not apply to moles as well? I was confused just because in the example he said conceptually we could say that in the chemical equation 2Na(s) + 2H2O(l) --> 2NaOH(aq) + H2(g) that it is balanced because there are two atoms of Na on the left and 2 atoms of Na on the right. But, he also said that there were 2 moles of Na on the left and 2 moles of Na on the right so I wasn't sure if the moles also had to be equal on the left and right sides?

[Bryan Cheng 1H]

You can think of it either way (with moles or with atoms). It's just the difference between the atomic scale and the larger molar scale -- you can think of the using moles instead of atoms as multiplying the quantity of everything in atoms by Avogadro's number. The effect is the same, so whichever is easier for you to think about!

[Emily Chen 1G]

Just for clarification, the number of moles of a particular element, in this case Na, would have to be the same on both sides of the reaction, but the TOTAL number of moles on the reactants and products side do not necessarily have to equal to each other

[Vincent Nguyen 3G]

The thing about moles is that the amount of moles you have can change depending on how bonds form. For the sake of conceptualizing this, pretend we had a hypothetical equation like 2H2(g) + O2(g) --> 2H2O(g). There are 3 moles on the left, but only 2 on the right. This is because the 2H2 and single O2 (2+1 moles) are bonding together to form 2H2O (2 moles). There are still the same amount of atoms (which is all that matters for balancing), but there are fewer moles because the 2H2 and O2 have bonded together.

[Julia Zahra]

The number of moles of individual elements must stay the same but not the number of moles of different molecules that have multiple elements.

[Maddie Klee 3K]

The easiest way for me to think about these problems is to focus on each element to make sure that there are the same number of moles of that element on each side of the equation. The conservation of mass just means that mass cannot be created or destroyed, so you balance the reaction to ensure that the atomic mass of the reactants is equal to that of the products. This doesn't specifically relate to moles, but you can think of it in terms of the moles of a specific element need to be the same on both sides, but not necessarily the moles of other elements.

[605721741]

Atoms must be balanced in chemical reactions. There are for moles of hydrogen on the left. There are 2 moles of NaOH and one mole of H2 on the right. This means 2 hydrogen atoms from NaOH and 2 from H2. This means 4 hydrogen atoms, which creates a balanced equation.

[Joshua Lee 3C]

The law of conservation of mass simply states that in a reaction, matter cannot be created or destroyed. In this situation, this holds true even if there are fewer total moles of the product than the reactant. This is because the individual number of moles of each element remains the same. There are four moles of hydrogen before and after the reaction. There are 2 moles of sodium before and after the reaction. There are 2 moles of oxygen before and after the reaction. The only reason that the number of total moles changed is due to the way the elements bonded with each other. If a more complex molecule is formed, then there may be fewer moles because one mole of the complex molecule has a lot more mass than the simpler compounds prior to the reaction. The number of total grams before and after the reaction still remains the same. Thus, the law of conservation of mass was not broken in this situation.

[Edwin Montalvo 1G]

The easiest way to see this is to understand the relationship between an atom and a mole. Both of these are quantity. An atom is typically the smallest quantity we use in chemistry because we can draw particles and observe them. A mole is just a bunch of atoms. In fact, a mole is 6.022 x 10^23 particles (or in this instance, atoms). Therefore, we can choose to view the chemical formula in terms of atoms because we need to start and end with the same amount of atoms, which in turn means that the mass on both sides is conserved. So, just remember that these are quantities, and viewing them as atoms helps visualize the balance.