I hear you, and maybe I have a possible answer. When we teach writing at HMC, we are told by the head writing facilitator that as students grapple with "thesis" and "argument," they lose some of the more basic skills like "spelling and grammar." This is known as cognitive overload. I found a nice overview of this theory as it pertains to education here. Maybe what is happening is that as students grapple with new chemistry concepts, they lose their ability to do arithmetic. 

The other option is that kids today really aren't learning this kind of thing in K-12 education, and I have reasons to believe based on my own students that this is more true than we might like. 

I don't know the answer to "How," but at least I might an an explanation for "why."

If the student has been admitted to the college and has registered for a class, then he/she was deemed competent to study chemistry in that class. If one or more has slipped through the cracks and is not competent, then it is the instructor's responsibility to try to bring the student up to speed. If this is a frequent occurrence and is hampering the learning of chemistry, then it might be a sytemic problem that should be addressed by the department and/or college. In that case, perhaps remedial courses are needed, or the admissions process needs to be tightened up to prevent unprepared students from entering the college and taking the class.

I agree with Dr. Maxell. However, the student we have in class are a function of the college admissions process, prereqs for the course, etc., and these can and should be adjusted as needed. In the meantime, we MUST teach our students what they need, even if it seems remedial. This may take additional student/staff time until the situation is addressed in the admissions process.

I wrote this response rather quickly, so I apologize for any grammatical or other errors.

I run into the same kind of thing -- a lot.  Completion of college algebra is a prerequisite to the course, but many students seem to struggle with basic rearrangement of equations.  I have a one question placement exam to determine whether a student should go into non-majors chemistry or general chemistry if I don't have information on their math preparation:
PV=nRT, what does n=?

I fear that many of the people who have the college algebra prerequisite still cannot answer that question.  Of course we should teach the students we have, but how does one catch up a student that struggles with the basic math to the level required for the course, plus all the chemistry content?  Their math level is ~eighth grade and it's unreasonable to expect them to get five years of math plus all the chemistry content in a semester.

My answer for next year is to make Supplemental Instruction (additional time led by upper level students) mandatory and require them to purchase the Calculations in Chemistry book and work through it with peer leaders for an hour each week.  I'm not sure how much it will help, but it's the only idea I currently have.

I taught Gen Chem I during the Spring and Fall 2018 semesters. In both semesters, about 25% scored less than 50% on the first exam, and just under 25% scored 80+ percent. No longer do those of us teaching Gen Chem I at Illinois State see a traditional bell curve, but we see a double curve or a misshapen bell curve with the low end being skewed larger. The last three semesters I taught Gen Chem I (Fall 2015, Spring 2018, Fall 2018), I have given a 'bonus exercise' in the form of clicker questions that are for extra credit added to your semester total about study habits. I have given it after the second exam (traditionally the lowest score of the 4 hour exams) and simply ask that they answer honestly and tell them that I will not match answers to students. I asked 4 questions the first time, then 5 the next two. I started with the question "How  many times have you visited  Dr. Hamaker’s office for help?" to gauge honesty. I know about how many have been to my office, and the self-reported numbers are always high (more visits for help than have actually occured). I as 2 or 3 other questions about help (lab instructor office hours; Chemistry Resource room hours; other tutoring).

The last question is always "Approximately what percentage of the suggested end-of-chapter problems have you done?" Fall 2015, 19.4% self reported doing NONE; 73.1% reported doing less than 60% overall. In Spring 2018, 18% reported none, 82% less than 60%. In Fall 2018, 21% reported none; 80% less than 60%. I don't know if not doing the problems is because they can't do the math, or if struggling with the math when working problems is because they don't practice doing problems. I can say that teaching Gen Chem II for the first time ever this semester, I saw some really strange math on my students' RICE tables for the equilibrium problem.

Interesting comments.
The same thing also happens to me, and I'm on the other side of the ocean. I thought it was a problem of the educational system in my country, but I see that it is global. And my question is, What is happening? and How can we solve it?
I think it's a problem that involves the whole educational system, from kindergarten.

At last summer's Biennial Conference on Chemical Education at Notre Dame, I heard more than one speaker raving about the book, "Teach Students How to Learn: Strategies You Can Incorporate Into Any Course to Improve Student Metacognition, Study Skills, and Motivation" by Saundra McGuire.  I'd seen a webinar with Dr. McGuire a few years ago, and she's very inspriring. 

I have a copy of the book now, but only got about halfway through it in the first half of September before the semester took over my life. But her points mirror those that Chris is making. I think many of our students don't realize what they need to be doing to study effectively and how much effort / time it will require.  (Or not - one of Dr. McGuire's points is to utilize short bursts of focused studying.) 

From what I've seen, students who don't do well in general chem are either completely overburdened with other courses, work, family or they're unmotivated for some reason (mental health, general disengagement with college) or they're trying super hard and falling short.  It's this last group that the book is most useful for reaching, I think. 

Hopefully in May and over the summer, I'll have more time to finish reading it and put together an action plan for next fall's gen chem class!

We've seen the same trend.  Our school does employ SI led by students (which has helped), and we also have an additional 1 credit workshop course in which students work through additional problems in small groups (similar to a recitation).  The workshop is open to all students, but is mandatory for any incoming student with a Math ACT score below a threshold level.  The workshop helps, but we had to tweak our approach (and still need to) to maximize the impact.

I agree that students generally lack the understanding/desire to study and engage with the material in a meaningful manner - both can be addressed, but it takes a time investment on our part which may or may not be doable depending on what else is cooking that semester.