38 votes
Accepted

How can there be decimal subscripts in a molecular formula?

First of all, in chemistry two types of formulas exist: structural and empirical. A structural formula shows the way atoms are connected. An empirical formula only summarizes atoms and their ratios. ...
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  • 20.6k
25 votes

Do coefficients matter in rate law?

This confusion pops up often around here. The problem usually comes down to insufficient consideration of units. When you say "rate", what do you mean? It has units of mol per volume per second, ...
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  • 20.2k
25 votes

Why can't I balance this reaction algebraically?

Here's how to solve the system in python: ...
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  • 20.2k
22 votes
Accepted

Are matrices a good way to do stoichiometry?

Sure, most of the equation-balancing Javascript calculators on the web use that method. It's overkill to balance a single equation that way, but if you're automating the process or if you have a lot ...
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  • 5,457
19 votes
Accepted

CH4 + O2 => CO2 + 2H2, what's wrong with it?

The reaction as you state it is correct only if there will react only one molecule of oxygen. But the reaction describes burning of methane which is supposed to be in the presence of excess of oxygen. ...
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19 votes
Accepted

Why can't I balance this reaction algebraically?

Given the reaction $\ce{a Cu + b HNO3 -> c Cu(NO3)2 + d NO + e H2O}$, we can set up four equations: $$\begin{align} a &= c & &\text{(for Cu)} \tag{1} \\ b &= 2e & &\text{(...
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  • 21.4k
19 votes

What is the mathematical meaning of the plus sign (+) in chemical reaction equations?

I think we are over-interpreting the poor $+$ sign. In older books you will also see an $=$ sign instead of an arrow just like what Lavoisier did. We have to get hold of La Traité Élémentaire de la ...
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  • 31.9k
16 votes

What is the analogue of "molecule" for ionic compounds?

The term you are looking for is formula unit, I think. Wikipedia doesn't really describe it super well, but just to give an example, you could write the sentence $\pu{58.44 g}$ of $\ce{NaCl}$ ...
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  • 65.6k
15 votes
Accepted

Simplest way to balance any chemical equation

You can do this in the 'guided trial-and-error' method that LordStryker showed which is probably quickest for simple reactions, or approach it in a purely mathematical fashion which is the method I ...
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  • 6,570
15 votes

Can a chemical reaction have two balanced equations?

You can expand the second equation so it has the same amount of educt on the left side as the first one: $$\ce{6XeF4 + 12H2O → 3Xe + 3XeO3 + 24HF + 1.5 O2}$$ Difference on the product side, compared ...
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  • 11.4k
14 votes
Accepted

How to calculate valence factor (n-factor) for a element?

Hint : n-factor of a molecule/compound is defined as the change in oxidation state per molecule. You have correctly calculated the change of one carbon atom as 4. But how many carbon atoms are ...
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  • 2,107
14 votes
Accepted

Is there a better way balancing reaction equations than systematic trial and error?

When using matrices, you are really just solving a system of equations. You can use any technique you know to solve the system of equations that a chemical equation represents. Using matrices is only ...
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  • 156
14 votes

What's the difference between reactant, excess reactant and limited reactant in a balanced equation?

Suppose that you want to make a cake. You would first need to go your fridge to check whether you have the necessary ingredients to make one. So you go to your fridge. Luckily, you have all the ...
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  • 4,200
13 votes

Number of atoms in NaCl unit cell

The picture you showed does have an unequal number of sodium cations and chloride anions. However, the picture shows only part of a crystal. Every atom that is on a boundary of the shown cube, ...
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  • 20.2k
13 votes

Do coefficients matter in rate law?

The rate of production of $\ce{B}$ is given by: $$\frac{\mathrm{d}[\ce{B}]}{\mathrm{d}t} = k[\ce{A}]^2$$ The rate of consumption of $\ce{A}$ is given by: $$\frac{\mathrm{d}[\ce{A}]}{\mathrm{d}t} = -...
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  • 65.6k
13 votes

Unknown notation relating to stoichiometry

It is probably supposed to be the 'corresponds to' sign[ref, p 105]; in this case simply representing that for every two moles of $\ce{NaOH}$ one mole of $\ce{H2SO4}$ is converted by reaction ...
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13 votes
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What does n(Na2CO3) mean?

Yes, the standardized quantity symbol according to ISO 80000-9:2009 Quantities and units — Part 9: Physical chemistry and molecular physics as well as the recommended quantity symbol according to ...
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  • 2,228
12 votes
Accepted

Is this definition of mole correct?

The mole is a base unit as specified in the Système international d’unités (SI) by the bureau international des poids et mesures. Its decisive definition is that published in French: La mole est ...
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  • 63.6k
12 votes
Accepted

Unknown notation relating to stoichiometry

The sign $≏$ is read "is chemically equivalent to". The term chemically equivalent refers to a specific chemical reaction. For the reaction $$\ce{N2(g) + 3 H2(g) -> 2NH3(g)}$$ $$\pu{1 mol } \ce{N2} ...
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11 votes

More than one way of balancing a chemical equation

This happens when you have a sum of two (or more) independent reactions. You can balance each one, and then add them together in an arbitrary proportion. People usually run into this when trying to ...
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  • 29.6k
11 votes

What is the mathematical meaning of the plus sign (+) in chemical reaction equations?

Here is an interpretation I came up for it, so suppose you have the molecule $\ce{CH4}$, then we can think of this as a vector as $ \langle 1,4 \rangle$ where the $1$ stands for number of carbons and $...
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10 votes

Determine what the unknown element is

The mass of the sulfate is $m_{\ce{SO4}} = 0.610~\mathrm{g}$, the mass of the metal is $m_\text{M} = 1.250~\mathrm{g}$. With the molar mass of the sulfate anion $M_{\ce{SO4}} = 96.061~\mathrm{g\, mol^...
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  • 10.5k
10 votes
Accepted

How much sulphuric acid can be obtained from 5 kg of sulfur

$\ce{S6}$, $\ce{S8}$, $\ce{S12}$ – does it make a difference or is it just a trick to make the question more complicated than it is? What you know for sure is: $M(\ce{S}) = 32.065\ \mathrm{g}\cdot \...
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10 votes

After completing an experiment, a student finds that the percentage yield exceeded 100%. What could have occurred to the sample to lead to this?

Below is a basic example. If percent yield is based on a mass, then a >100% yield can result from a product contains materials that should not be there. For instance, a product that should be ...
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  • 4,549
10 votes

What is the difference between ''1 C atom and 1 mole of C atoms?''

What is the difference between one egg and one dozen eggs? A dozen is simply a certain count, in this case 12, that is rendered as one unit for our convenience. Avogadro's number s just the same ...
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  • 38.1k
9 votes
Accepted

When are we justified in making assumptions in questions

You can make certain assumptions in these and similar problems because concentration of uniform substances (whether it's expressed as molarity, molality, mass density, or something else entirely) is ...
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  • 11.6k
9 votes
Accepted

After completing an experiment, a student finds that the percentage yield exceeded 100%. What could have occurred to the sample to lead to this?

Preface: The problem is that you have overstated your percentage yield, and the symptom is that it is above 100%. Example: You and your friend each do an experiment where the literature states the ...
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  • 206
9 votes
Accepted

Error in formula for the solution to a chemical problem

The equation is a little dubious, as I mentioned in the comments. How do we come up with coefficients of this sort? Let's go from scratch. We don't know the coefficients at this point so we'll ...
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  • 65.6k
9 votes
Accepted

Balancing the reaction equation for the oxidation of lead(II) sulfide with ozone

This is a semi-standard example of why 'naive' balancing does not work. Ozone is a source of atomic oxygen, producing free oxygen molecules. So, in 'mild' conditions only one oxygen per ozone ...
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  • 20.6k
9 votes

Why can't I balance this reaction algebraically?

You seem to have had the right idea of fixing the scale by arbitrarily choosing the value of one coefficient, and then solving for the rest. Apparently, you just got stuck at some point, presumably ...
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